Biggest Moments in Military History From the Last 100 Years

The world's mightiest military came from humble beginnings as colonial militias representative of the era's free, white society. Men between the ages of 16 and 60 were recruited to colonial militia service from all walks of life, including shopkeepers, tutors, small farmers, and smiths. Left out of that recruitment were college enrollees, slaves, most free blacks, and clergy—and, in Virginia, Catholics. Men who were recruited were asked to make inarguably heavy sacrifices during their service, which included operations against Native Americans and supplementing Red Coats in border skirmishes with neighboring European colonies. The colonial militia's first overseas foray came in 1741 and ended in abject disaster when 4,000 American reinforcements joined an attempted British invasion of Cartagena, Colombia, then a Spanish possession. The invasion failed miserably and only around 600 American volunteers returned home alive from the expedition.

In the lead up to the Revolutionary War, the American militia was prepared to step up in case of emergency for the paid, trained soldiers in the Continental Army (established by the Continental Congress in 1775), although the militia ultimately provided far more soldiers for that effort than the fledgling army. The British Regulars, or Red Coats, assumed this growing military was made up of laborers, criminals, and other struggling members of society, and ill-equipped to handle the brutality of war. That misguided perspective—largely brought about because Britain itself enlisted soldiers from its own, lowest classes—along with a big boost from the French, ultimately cost Britain the colonies during the Revolutionary War.

The U.S. Armed Forces over the last century have played major roles in two world wars, a wide variety of civil conflicts, and dozens of ongoing military campaigns. These efforts have made significant impacts on how our government makes decisions that may affect domestic and foreign affairs. The military itself has undergone a few structural changes in that time as well, including adding new divisions and permitting women and LGBTQ+ people to serve in all military branches.

Stacker looked at information from the Defense Manpower Data Center, the U.S. Census historical population tables, and the St. Louis Federal Reserve to see how the military has changed over the years. By comparing data sets (last updated 2019) we were able to determine the percentage of Americans enlisted in the military and the number of Americans in each military branch every year from 1917 to 2019.

You may also like: 50 ways the military has changed in the last 50 years

Woodrow Wilson
Woodrow Wilson (1856 - 1924), the 28th president of the United States (1913 - 1921), points with his finger while making a speech from a platform on his campaign trail, Virginia. Hulton Archive/Getty

1917: U.S. enters WWI

- Army strength: 421,467 people
- Navy strength: 194,617 people
- Marine Corps strength: 27,749 people
- Air Force strength: Not yet formed
- Total strength: 643,833 people
- Percent of population enlisted: 0.62%

Congress granted President Woodrow Wilson's request for a declaration of war against Germany on April 6, 1917, officially entering the United States into World War I. The Army expanded dramatically in the next 18 months, from 200,000 men in December 1916 to 3,685,000 troops in 1918—2 million of whom were stationed in France to serve in Gen. John Pershing's American Expeditionary Force.

1918: Meuse-Argonne Offensive

- Army strength: 2,395,742 people
- Navy strength: 448,606 people
- Marine Corps strength: 52,819 people
- Air Force strength: Not yet formed
- Total strength: 2,897,167 people
- Percent of population enlisted: 2.81%

Thousands of American troops joined forces in September 1918 with the allied intervention force at Archangel in response to the Bolshevik revolution in Russia. American soldiers engaged in several major battles that year as part of World War I, including the Meuse-Argonne Offensive from Sept. 26 through Nov. 11, which involved more than 1 million American soldiers (26,000 of whom died in battle and 120,000 casualties overall). The offensive was the largest of the American Expeditionary Forces in World War I and is largely credited with ushering in the end of the war on Nov. 11, 1918.

Treaty of Versailles
Government Officials Drafting the Terms of the Treaty of Versailles. Getty

1919: Treaty of Versailles

- Army strength: 851,624 people
- Navy strength: 272,144 people
- Marine Corps strength: 48,834 people
- Air Force strength: Not yet formed
- Total strength: 1,172,602 people
- Percent of population enlisted: 1.12%

Following the conclusion of WWI, the Treaty of Versailles was signed June 28, 1919, in the Palace of Versailles, France. The peace document included signatures from allied powers and Germany, and went into effect the following year with redrawn German boundaries and an outline of required reparations from the country. After signing the treaty on behalf of the United States and presenting his Fourteen Points that included the formation of the League of Nations, U.S. President Woodrow Wilson returned home only to find an obstinate Senate had voted against the treaty—twice.

[Pictured: The signing of the Treaty of Versailles on June 28, 1919]

1920: National Defense Act amended

- Army strength: 204,292 people
- Navy strength: 121,845 people
- Marine Corps strength: 17,165 people
- Air Force strength: Not yet formed
- Total strength: 343,302 people
- Percent of population enlisted: 0.32%

Congress in 1920 passed an amendment to the National Defense Act, which rejected the concept of an expandable "Regular Army" and called for the U.S. Army to have three main divisions: the standing Regular Army, National Guard, and Organized Reserves.

1921: The Unknown Soldier

- Army strength: 230,725 people
- Navy strength: 132,827 people
- Marine Corps strength: 22,990 people
- Air Force strength: Not yet formed
- Total strength: 386,542 people
- Percent of population enlisted: 0.36%

Congress approved the burial of an unidentified body from World War I on March 4, 1921, at Arlington National Cemetery. The "Unknown Soldier" commemorates the 116,516 American soldiers killed in World War I, many of whose bodies were never identified.

1922: Washington Naval Treaty

- Army strength: 148,763 people
- Navy strength: 100,211 people
- Marine Corps strength: 21,233 people
- Air Force strength: Not yet formed
- Total strength: 270,207 people
- Percent of population enlisted: 0.25%

The Washington Naval Treaty was signed by the United States, United Kingdom, Japan, France, and Italy on Feb. 6, 1922. The document, also known as the Five-Power Treaty, was drafted to prevent an arms race following World War I.

1923: Warlordism

- Army strength: 133,243 people
- Navy strength: 94,094 people
- Marine Corps strength: 19,694 people
- Air Force strength: Not yet formed
- Total strength: 247,031 people
- Percent of population enlisted: 0.22%

American soldiers spent part of 1923 in China, helping to control unrest that ensued amidst warlordism—the era from 1923 to 1928 marked by the dilemma of Beiyang Army military factions vying for control of China. The period of time represents a division of control spread out across the country.

1924: First U.S. occupation of Dominican Republic ends

- Army strength: 142,673 people
- Navy strength: 98,184 people
- Marine Corps strength: 20,332 people
- Air Force strength: Not yet formed
- Total strength: 261,189 people
- Percent of population enlisted: 0.23%

The U.S. Navy invaded the Dominican Republic in 1916, taking over the army, police, and several vital locations as Desiderio Arias, the Dominican Republic's secretary of war, was forced out of Santo Domingo. The occupation lasted through 1924, when waning public support following World War I and significant opposition internationally and among Dominicans inspired the U.S. to turn policing authority over to the Guardia Nacional. Back home, the Immigration Act of 1924 allowed immigration visas for 2% of the number of people from each nationality in the U.S. according to the 1890 census. The act neglected to include visas for any immigrants from Asia, making it illegal for them to cross the U.S. shoreline and enter in spite of continued unrest overseas.

1925: Riots in Shanghai

- Army strength: 137,048 people
- Navy strength: 95,230 people
- Marine Corps strength: 19,478 people
- Air Force strength: Not yet formed
- Total strength: 251,756 people
- Percent of population enlisted: 0.22%

Continued unrest from Chinese factions competing for political power in Shanghai erupted into riots. American troops were brought in to protect the public—and the terms of the Shanghai International Settlement during one of the most pivotal moments in Chinese history.

1926: U.S. squashes Nicaraguan coup d'état

- Army strength: 134,938 people
- Navy strength: 93,304 people
- Marine Corps strength: 19,154 people
- Air Force strength: Not yet formed
- Total strength: 247,396 people
- Percent of population enlisted: 0.21%

The 1912 to 1933 U.S. occupation of Nicaragua was part of the "Banana Wars," a period of various military interventions, massacres, and actions by the U.S. throughout the Caribbean and Central America following the Spanish-American War's 1898 conclusion and the 1934 establishment of the Good Neighbor Policy. U.S. military presence in Nicaragua chiefly functioned to protect American business interests (soldiers kept unrest at a minimum on plantations the U.S. had a stake in and break apart any uprisings) and thwart any other would-be occupiers from constructing the Nicaraguan Canal. In 1925, the election of conservative President Carlos Solorzano helped form a coalition government. Thirteen years into their occupation, the U.S. Marines left Nicara. But by October of that year, Nicaraguan Gen. Emiliano Chamorro Vargas staged a successful coup d'état against Solorzano. Chamorro became president, but the U.S. did not recognize his rule; after a liberal revolt, the U.S. military in January 1926 sent gun-boats and troops back into the country and forced Vargas' resignation.

1927: 'China Marines' in Shanghai

- Army strength: 134,829 people
- Navy strength: 94,916 people
- Marine Corps strength: 19,198 people
- Air Force strength: Not yet formed
- Total strength: 248,943 people
- Percent of population enlisted: 0.21%

The 4th Regiment of the U.S. Marine Corps was sent to China in January 1927 to protect American citizens—and U.S. business interests—there in the midst of Shanghai's ongoing civil unrest. Called the "China Marines," these members of the military would remain in Shanghai through 1941. On April 12, 1927, 5,000 people from Generalissimo Chiang Kai-shek's military forces and the Nationalist Party Kuomintang staged a violent squashing of the Communist Party of China known as the Shanghai Massacre, or April 12 Purge. Hundreds of Communists were captured, tortured or executed, and the action touched off years of anti-communist violence coined the "White Terror."

1928: Lt. Schilt receives Medal of Honor

- Army strength: 136,084 people
- Navy strength: 95,803 people
- Marine Corps strength: 19,020 people
- Air Force strength: Not yet formed
- Total strength: 250,907 people
- Percent of population enlisted: 0.21%

First Lt. Christian F. Schilt between Jan. 6 and 8, 1928, took it upon himself to make 10 trips in his Vought O2U Corsair aircraft into Quilali, Nicaragua, in order to evacuate 18 U.S. Marines who had been injured in the conflict there. These takeoffs were accomplished amidst burning villages and extensive enemy fire, according to the U.S. Marine Corps, however, Schilt completed his mission, saved three lives in the process, delivered necessary provisions, and in June of 1928 was awarded the Medal of Honor.

1929: Cayes Massacre

- Army strength: 139,118 people
- Navy strength: 97,117 people
- Marine Corps strength: 18,796 people
- Air Force strength: Not yet formed
- Total strength: 255,031 people
- Percent of population enlisted: 0.21%

The U.S. occupation of Haiti began in 1915 as a means of keeping the country from German occupation that could disrupt passage through the Panama Canal. By 1929, there was deep resentment against Americans for their roles in censoring the press, collecting customs duties, being the distributor of essentials like food and medicine, and for forcing a new constitution upon the Haitians which. Uprisings were commonplace, including a 1918 guerrilla war that rose up against U.S.-imposed forced labor to build roads through the country. On Dec. 6, 1929, U.S. Marine battalions in Les Cayes opened fire on 1,500 protestors who were part of a national strike and area rebellion. Twelve Haitians were killed and 23 wounded in the massacre. Worldwide backlash from this violence helped to eventually inspire the end of the U.S. occupation there.

1930: London Naval Treaty

- Army strength: 139,378 people
- Navy strength: 96,890 people
- Marine Corps strength: 19,380 people
- Air Force strength: Not yet formed
- Total strength: 255,648 people
- Percent of population enlisted: 0.21%

France, Italy, the United Kingdom, Japan, and the U.S. signed the Treaty for the Limitation and Reduction of Naval Armament (commonly referred to as the London Naval Treaty) on April 22, 1930, to regulate submarine warfare and limit the building of naval ships. The terms of the treaty were put in place to avert a naval arms race following WWI and to build upon the Five Powers Treaty of 1922.

1931: Japan violates League of Nations

- Army strength: 140,516 people
- Navy strength: 93,307 people
- Marine Corps strength: 16,782 people
- Air Force strength: Not yet formed
- Total strength: 252,605 people
- Percent of population enlisted: 0.20%

The Great Depression's effects continued to spread throughout the world in 1931, increasing tensions at home and abroad. It would be years before the onset of WWII; however, a hint of ensuing international conflict set in when Japanese forces captured Manchuria in violation of the League of Nations in September 1931. The invasion put U.S. Secretary of State Henry L. Stimson in the position of determining a way to thwart the conflict. He issued the Stimson Doctrine in January 1932, which said the U.S. would not honor agreements or treaties between Japan and China that were in violation of existing U.S. rights or agreements.

1932: Military collides with Bonus Marchers

- Army strength: 134,957 people
- Navy strength: 93,384 people
- Marine Corps strength: 16,561 people
- Air Force strength: Not yet formed
- Total strength: 244,902 people
- Percent of population enlisted: 0.20%

In the summer of 1932, Congress denied military veterans an immediate bonus payment for participation in WWI. "Bonus Marchers" numbering more than 20,000 remained in Washington after this decision, demonstrating discontent so much that President Herbert Hoover called for the Army's help. Hundreds of troops intervened on the marchers' camps force that left two protesting World War II veterans dead and caused a massive outcry among the American public.


Civilian Conservation Corps.
Young African-American men working for the Civilian Conservation Corps dig holes to build a fence in Greene County, Georgia, United States, May, 1941. From the New York Public Library. Smith Collection/Gado/Getty

- Army strength: 136,547 people
- Navy strength: 91,230 people
- Marine Corps strength: 16,068 people
- Air Force strength: Not yet formed
- Total strength: 243,845 people
- Percent of population enlisted: 0.19%

A little over three years after the initial stock market crash, the unemployment rate in the U.S. was 24.9%, with almost 15 million U.S. citizens jobless. In response, President Franklin D. Roosevelt started the Civilian Conservation Corps (CCC) as part of his New Deal program. The CCC comprised existing government departments, including the Army, administering voluntary camps that provided more than 3 million men with manual labor jobs, mainly in natural resource conservation. Many Army officers benefitted from overseeing the camps, not otherwise having the opportunity to supervise large groups of personnel in the period between the world wars.

Also in 1933, Adolf Hitler rose to power in Germany.

1934: U.S. occupation ends in Haiti

- Army strength: 138,464 people
- Navy strength: 92,312 people
- Marine Corps strength: 16,361 people
- Air Force strength: Not yet formed
- Total strength: 247,137 people
- Percent of population enlisted: 0.20%

After President Franklin D. Roosevelt established an agreement to disengage from Haiti in 1933, the U.S. occupation of the country officially drew to a close Aug. 1, 1934. This followed FDR's "Good Neighbor" policy, which meant not interfering in Latin American domestic affairs; however, the U.S. did not relinquish its hand in Haitian finances until 1947

1935: GHQ Air Force and B-17

- Army strength: 139,486 people
- Navy strength: 95,053 people
- Marine Corps strength: 17,260 people
- Air Force strength: Not yet formed
- Total strength: 251,799 people
- Percent of population enlisted: 0.20%

Buoyed by advances in aviation and the use of planes in World War I, more than a dozen commissions and boards grappled from 1919 and 1934 to determine how to move forward with military aviation in a way that could be separate from general support aviation by the Army. The Baker Board of 1934 proposed a peacetime air force, established in 1935 as the General Headquarters (GHQ) Air Force and based at Langley Field, Va. The GHQ Air Force would function as a centralized unit for long-distance strikes. As part of this effort, 1935 also marks the unveiling by the Boeing Airplane Co. of the Model 299, a long-range, four-engine heavy bomber that would transform air battles that was eventually coined the "B-17 Flying Fortress."

1936: Abraham Lincoln Brigade

- Army strength: 167,816 people
- Navy strength: 106,292 people
- Marine Corps strength: 17,248 people
- Air Force strength: Not yet formed
- Total strength: 291,356 people
- Percent of population enlisted: 0.23%

International governments looked upon the Spanish Civil War with trepidation, worried escalation could lead to a larger-scale, world war. To prevent higher stakes, a number of Western governments including the United States signed a non-intervention treaty. That didn't stop roughly 35,000 volunteers from 50 countries—including 2,800 U.S. volunteers—from traveling to Spain in 1936 to fight for the Spanish Republic against a fascist uprising aided by Hitler and Mussolini. American volunteers named their units the John Brown Battery, Abraham Lincoln Battalion, and George Washington Battalion. These units, along with volunteers from Britain, Canada, and several other countries, formed the Fifteenth Unit Brigade, known colloquially as the Abraham Lincoln Brigade.

1937: Protective Mobilization Plan

- Army strength: 179,968 people
- Navy strength: 113,617 people
- Marine Corps strength: 18,223 people
- Air Force strength: Not yet formed
- Total strength: 311,808 people
- Percent of population enlisted: 0.24%

As the U.S. began to concentrate its economic power toward mobile war operations, the government enacted the Protective Mobilization Plan, in which the National Guard would be inducted into federal service. This gave the Army a protective force of 400,000 troops, which could protect the nation while the Army focused on expansion and training. These plans were the foundation for the mobilization of troops in the summer of 1940, not long before the U.S. entered WWII.

[Pictured: Soldiers from the 118th Infantry Regiment, 30th Infantry Division, South Carolina National Guard]

1938: Munich Agreement inspires U.S. hemisphere defense strategy

- Army strength: 185,488 people
- Navy strength: 119,088 people
- Marine Corps strength: 18,356 people
- Air Force strength: Not yet formed
- Total strength: 322,932 people
- Percent of population enlisted: 0.25%

The Munich Agreement (also called the "Munich Betrayal") in September 1938 was a settlement between Italy, Germany, and France allowing Germany to annex the Sudetenland in western Czechoslovakia, just four months after it became public knowledge that Hitler intended to occupy all of that country. The pact prevented all-out war but ceded a significant part of Czechoslovakia to German rule. As the American military watched things unfold overseas and noted the advances worldwide in war technology and artillery reach, it adjusted its defense strategy to focus on protecting the Western Hemisphere from enemy fire and hostile air bases in what has been coined the "hemisphere defense."

World War II Ft. Bragg
circa 1942: Group view of the soldiers of the 41st Corps of Engineers, an African-American army battalion, standing in formation and holding the American flag, Fort Bragg, North Carolina, World War II (1939-1945). African-American soldiers still fought in segregated units during World War II. Hulton Archive/Getty

1939: World War II officially begins

- Army strength: 189,839 people
- Navy strength: 125,202 people
- Marine Corps strength: 19,432 people
- Air Force strength: Not yet formed
- Total strength: 334,473 people
- Percent of population enlisted: 0.26%

World War II officially began with Germany's invasion of Poland, followed two days later by the U.K. and France declaring war on Germany. Although the U.S. remained neutral until 1941, the military was still taking measures to train and plan for mobilization. FDR in 1939 proposed the Cash and Carry policy, which replaced the 1936 Neutrality Acts and allowed the sale of military technology and materials to nations at war.

1940: U.S. prepares for war

- Army strength: 269,023 people
- Navy strength: 160,997 people
- Marine Corps strength: 28,345 people
- Air Force strength: Not yet formed
- Total strength: 458,365 people
- Percent of population enlisted: 0.35%

American involvement in WWII was limited: The military contributed materials and financial support to Great Britain, the Republic of China, and the Soviet Union. Expecting eventual involvement, however, the U.S. was also beginning to strengthen its own military forces. From June of 1940 through December of 1941, $36 billion was allocated to the War Department (more than was spent on all of World War I) and $8 billion for the Army alone. Meanwhile, the munitions program was preparing weaponry to sustain a military force of more than 1 million men.

1941: U.S. enters WWII

- Army strength: 1,462,315 people
- Navy strength: 284,427 people
- Marine Corps strength: 54,359 people
- Air Force strength: Not yet formed
- Total strength: 1,801,101 people
- Percent of population enlisted: 1.35%

The U.S. declared war on Japan on Dec. 8, 1941, following the Japanese bombing of Pearl Harbor. Japan's attack lasted just one hour and 15 minutes but managed to inflict heavy damage upon the U.S. Pacific fleet, including killing 2,403 people (68 of whom were civilians), disabling 112 watercraft and 164 aircraft, and sinking six battleships. The USS Arizona is still on the ocean floor of Pearl Harbor with the entire crew on board.

Three days after declaring war on Japan, the U.S. declared war on Germany and Italy.

1942: War with Bulgaria, Hungary, and Romania

- Army strength: 3,075,608 people
- Navy strength: 640,570 people
- Marine Corps strength: 142,613 people
- Air Force strength: Not yet formed
- Total strength: 3,858,791 people
- Percent of population enlisted: 2.86%

The U.S. officially declared war on Bulgaria, Hungary, and Romania on June 5, 1942. The American government spent colossal amounts of money on the effort and assumed a wartime economy. American businesses, farmers, and factory workers all contributed. Toward the end of the year, it was mandatory for all men between the ages of 18 and 64 to register for the draft, though many volunteered before being called. U.S. troops inflicted critical damage on a Japanese naval fleet in the Battle of Midway in June 1942.

With involvement in World War II escalating, rationing of foods such as sugar, meat, and coffee began.

Dwight Eisenhower
England- General Dwight D. Eisenhower, Supreme Commander of the Allied Expeditionary Forces, gives the order of the day "Full Victory--Nothing Else" to paratroopers somewhere in England. These first assault troops, faces blackened, boarded their planes immeditately after for the flight across the Channel. Getty

1943: Eisenhower chosen to lead Allies

- Army strength: 6,994,472 people
- Navy strength: 1,741,750 people
- Marine Corps strength: 308,523 people
- Air Force strength: Not yet formed
- Total strength: 9,044,745 people
- Percent of population enlisted: 6.61%

Gen. Dwight David Eisenhower in 1915 graduated from the U.S. Military Academy at West Point, just three years before World War I began. He worked up through the ranks from infantryman to captain, instructor, major, and lieutenant colonel before the first world war drew to a close. By the time the second war rolled around, Eisenhower had worked in varying capacities such as an instructor, football coach, and assistant to Gen. Douglas MacArthur; and graduated from Army War College, Fort McNair, Washington D.C., and first in his class from Command and General Staff School in Fort Leavenworth, Kansas. On Feb. 11, 1943, the future U.S. president was appointed Supreme Commander of Allied Expeditionary Forces to command the Operation Overlord, which kicked off by invading Italy that year.

1944: D-Day

- Army strength: 7,994,750 people
- Navy strength: 2,981,365 people
- Marine Corps strength: 475,604 people
- Air Force strength: Not yet formed
- Total strength: 11,451,719 people
- Percent of population enlisted: 8.27%

In the early morning of June 6, 1944, Allied troops enacted D-Day, which would bring the war about-face and bring down Nazi Germany. Approximately 155,000 Allied troops comprising U.S., British, and Canadian soldiers stormed Omaha Beach in Normandy, France.

1945: Battle of Iwo Jima

- Army strength: 8,266,373 people
- Navy strength: 3,319,586 people
- Marine Corps strength: 469,925 people
- Air Force strength: Not yet formed
- Total strength: 12,055,884 people
- Percent of population enlisted: 8.62%

Among dozens of significant battles throughout World War II was the Battle of Iwo Jima, during which time the Japanese island of Iwo Jima was taken over by the U.S. Marine Corps and Navy. After the invasion of Germany, WWII officially ended when President Harry Truman ordered the atomic bombs to be dropped on Japan in August 1945. Overall, the U.S. suffered almost half a million military and civilian casualties during the war, and the government spent about $4.1 trillion in modern dollars—making it the most expensive war in U.S. history.

1946: First session of the United Nations

- Army strength: 1,435,496 people
- Navy strength: 978,203 people
- Marine Corps strength: 155,679 people
- Air Force strength: 455,515 people
- Total strength: 3,024,893 people
- Percent of population enlisted: 2.14%

The U.S. military continued to demobilize in 1946, but slowed down the recovery of troops to fulfill outstanding obligations overseas. This decision resulted in protests from countries like China and France that did not subside until more than half of American troops returned home. The newly formed United Nations General Assembly had its first meeting in February 1946 at the Methodist Central Hall in London. The goal of the international organization was to encourage cooperation between countries and avoid another conflict like World War I and II. Its headquarters would be built in lower Manhattan in 1948, thanks to an $8.5 million donation from John D. Rockefeller to purchase a parcel previously occupied by a slaughterhouse.

1947: Air Force, National Security Council founded, Cold War erupts

- Army strength: 685,458 people
- Navy strength: 497,773 people
- Marine Corps strength: 93,053 people
- Air Force strength: 305,827 people
- Total strength: 1,582,111 people
- Percent of population enlisted: 1.10%

As post-war demobilization continued, the conversation of how the military would organize itself helped to create in 1947 the United States Air Force and National Security Council. The council spawned the Central Intelligence Agency and Department of Defense. Post-war tensions between the Soviet Union and U.S. had officially manifested as the Cold War. To contain the spread of communism, President Truman facilitated the Truman Doctrine March 29, 1947, which transferred $400 million to Turkey and Greece to aid in the fight against communism.

1948: Marshall Plan signed to rebuild Europe

- Army strength: 554,030 people
- Navy strength: 417,535 people
- Marine Corps strength: 84,988 people
- Air Force strength: 387,730 people
- Total strength: 1,444,283 people
- Percent of population enlisted: 0.98%

The Army's chief responsibilities as part of the National Security Act were to carry out land operations, prove anti-aircraft units, and supply occupation and security garrisons for use overseas. The Navy continued to control the Marine Corps, and the new Air Force commanded strategic air warfare and combat air backup for the Army. President Truman signed the Marshall Plan on April 3, 1948, which allowed the U.S. to provide more than $12 million to help rebuild western European economies. Congress passed a law declaring the Civil Air Patrol as the official civilian backup of the U.S. Air Force. Finally, on July 26, 1948, Truman passed Executive Order 9981 ending racial segregation in the U.S. Armed Forces.

1949: North Atlantic Treaty

- Army strength: 660,473 people
- Navy strength: 447,901 people
- Marine Corps strength: 85,965 people
- Air Force strength: 419,347 people
- Total strength: 1,613,686 people
- Percent of population enlisted: 1.08%

An amendment to the National Security Act in 1949 transformed the National Military Establishment into an executive entity, also known as the Department of Defense, comprising the Army, Navy, and Air Force. The North Atlantic Treaty was signed in April 1949, establishing the intergovernmental military alliance known as NATO. The Cold War installment known as the Revolt of the Admirals also took place in 1949, during which retired and active Navy admirals alike publicly disagreed with President Truman on the subject of strategic nuclear bombing as the primary mode of national defense. Additionally, a "Red Scare" broke out in the U.S. during which several public figures were named in an FBI report as Communist Party members, including Helen Keller and Dorothy Parker.

Korean War
Men of the 187th US Regimental Combat Team prepare to take a ridge position somewhere in Korea. Keystone/Getty

1950: Korean War begins

- Army strength: 593,167 people
- Navy strength: 380,739 people
- Marine Corps strength: 74,279 people
- Air Force strength: 411,277 people
- Total strength: 1,459,462 people
- Percent of population enlisted: 0.96%

The Korean War touched off on June 25, 1950, when roughly 75,000 North Korean People's Army soldiers crossed the boundary between North and South Korea. The war on the Korean peninsula would last for three years without ever being officially declared and cost 36,914 lives. Original estimates following the war's conclusion put the death toll at 54,260; a number revised to reflect that was actually the total of all U.S. military deaths around the world between 1950 and 1953.

1951: Second capture of Seoul, Treaty of San Francisco

- Army strength: 1,531,774 people
- Navy strength: 736,596 people
- Marine Corps strength: 192,620 people
- Air Force strength: 788,381 people
- Total strength: 3,249,371 people
- Percent of population enlisted: 2.10%

As the Korean War waged on, United Nations soldiers liberated Seoul for a second time in March 1951. Later that year, the U.N. and communist North Korean forces engaged in truce talks. A second Red Scare emerged when Ethel and Julius Rosenberg were convicted of conspiracy to commit espionage and received the death penalty. As part of the Treaty of San Francisco, 49 countries signed a peace treaty in 1951 with Japan.

1952: First hydrogen bomb detonated

- Army strength: 1,596,419 people
- Navy strength: 824,265 people
- Marine Corps strength: 231,967 people
- Air Force strength: 983,261 people
- Total strength: 3,635,912 people
- Percent of population enlisted: 2.31%

The Treaty of San Francisco went into effect in 1952, officially ending the U.S. occupation of Japan. As nuclear testing continued, the U.S. military successfully detonated the first hydrogen bomb as part of Operation Ivy. That nuclear test still stands as the fourth largest for all U.S. tests of that kind. Meanwhile, President Dwight Eisenhower traveled to Korea to explore methods of ending the war.

1953: Korean Armistice Agreement

- Army strength: 1,533,815 people
- Navy strength: 794,440 people
- Marine Corps strength: 249,219 people
- Air Force strength: 977,593 people
- Total strength: 3,555,067 people
- Percent of population enlisted: 2.22%

The U.N., China, and North Korea on July 27, 1953, collectively engaged in an armistice to end the Korean War and pull troops out of both countries. The agreement was signed by U.S. Army Lt. Gen. William K. Harrison Jr. from the U.N. Command Delegation and North Korean Gen. Nam II, representing North Korea and China.

Elsewhere, the CIA and British Secret Intelligence Service collaboratively overthrew democratically elected Iranian Prime Minister Mohammad Mosaddegh in a coup d'etat when they learned of his involvement with the communist pro-Soviet Tudeh Party.

1954: U.S. Air Force Academy established

- Army strength: 1,404,598 people
- Navy strength: 725,720 people
- Marine Corps strength: 223,868 people
- Air Force strength: 947,918 people
- Total strength: 3,302,104 people
- Percent of population enlisted: 2.03%

Despite having authorized hundreds of millions of dollars of military budget aid to Vietnam, President Eisenhower advised against U.S. assisting the French in its continued conflict with Viet Minh. Eisenhower also approved the establishment of the U.S. Air Force Academy in Colorado. The U.S. Army was investigated by Sen. Joseph McCarthy for allegedly not cracking down hard enough on communism.

1955: Navy helps evacuate Chinese Nationalist soldiers

- Army strength: 1,109,296 people
- Navy strength: 660,695 people
- Marine Corps strength: 205,170 people
- Air Force strength: 959,946 people
- Total strength: 2,935,107 people
- Percent of population enlisted: 1.77%

As the conflict in Vietnam escalated, President Eisenhower sent U.S. advisers to South Vietnam when Prime Minister Ngo Dinh Diem received threats from multiple domestic sources. Also that year, the U.S. Navy helped the Republic of China evacuate Chinese Nationalist soldiers and residents from the Tachen Islands when the People's Liberation Army overtook the area. The Formosa Resolution detailed U.S. protection of the Republic of China from the People's Republic of China.

1956: Hungarian Revolution

- Army strength: 1,025,778 people
- Navy strength: 669,925 people
- Marine Corps strength: 200,780 people
- Air Force strength: 909,958 people
- Total strength: 2,806,441 people
- Percent of population enlisted: 1.66%

The Hungarian Revolution of 1956 began with an organized group of student protesters marching through the streets of Budapest on Oct. 23 with loudspeakers, chanting, "This we swear, this we swear, that we will no longer be slaves." After reading an anti-communist proclamation demanding an independent Hungary, students stormed the radio building near the Hungarian Parliament, prompting police to open fire. The violence killed one student and marked the first bloodshed in the revolution that ultimately toppled the Soviet government.

The violent outbreak turned the protest into an all-out nationwide revolt against Soviet policies that were being forced on the public by the Hungarian People's Republic. The uprising exploded with militias fighting the military, arming prisoners, and a death toll that included 2,500 Hungarians and 700 Soviet troops. Another 200,000 Hungarians fled as refugees. Meanwhile in the United States, officials and military leaders stood by watching events unfold, too afraid of another world war (or disrupting recently improved diplomacy with Moscow) to step in and assist the protestors. The revolution was squashed Nov. 4 when the Soviet Union sent soldiers into Hungary, causing an uproar by U.S. citizens and Hungarian demonstrators over the controversial lack of action by the U.S., which had gone so far as to utilize the CIA-run Radio Free Europe (RFE) to broadcast encouragement to the rebels, imply the U.S. would be arriving with help, and offer tactical insights for fighting the Soviets.

Ultimately, the Soviet Union would not regain governmental control over Hungary again.

1957: Distant Early Warning Sign

- Army strength: 997,994 people
- Navy strength: 676,071 people
- Marine Corps strength: 200,861 people
- Air Force strength: 919,835 people
- Total strength: 2,794,761 people
- Percent of population enlisted: 1.63%

Although the U.S. and Soviet Union vowed to proceed in the Cold War without engaging in full-on nuclear confrontation, the U.S. partnered with Canada to begin construction on the Distant Early Warning Line in 1954. This radar network, which became functional in 1957, was able to detect early signs of missile or air attacks coming from the north.

1958: Lebanon crisis

- Army strength: 898,925 people
- Navy strength: 639,942 people
- Marine Corps strength: 189,495 people
- Air Force strength: 871,156 people
- Total strength: 2,599,518 people
- Percent of population enlisted: 1.49%

The Lebanon Crisis broke out in 1958, caused by the threat of civil war between Maronite Christians and Muslims. U.S. Marines helped diffuse the situation as part of Operation Blue Bat, to help strengthen the pro-Western government under President Camille Chamoun and protect it from Syrian and Egyptian threats.

1959: U.S. Special Forces train soldiers in Laos

- Army strength: 861,964 people
- Navy strength: 625,661 people
- Marine Corps strength: 175,571 people
- Air Force strength: 840,435 people
- Total strength: 2,503,631 people
- Percent of population enlisted: 1.41%

Cold War tensions continued to mount when a Communist group in Laos overtook several provinces bordering North Vietnam and China. Neither side prevailed, despite the U.S. military intervention, training of Laotian soldiers, and growing presence there since 1957 in support of Royal Lao Armed Forces (known commonly by its French acronym, FAR).

1960: U.S.-Cuba standoff

- Army strength: 873,078 people
- Navy strength: 616,987 people
- Marine Corps strength: 170,621 people
- Air Force strength: 814,752 people
- Total strength: 2,475,438 people
- Percent of population enlisted: 1.37%

The U.S. acknowledged Fidel Castro as the leader of Cuba, but as Castro—who rose to power in 1959 following an armed uprising that unseated Cuban dictator Fulgencio Batista—increasingly nationalized U.S. companies and investments there, harsh penalties were enacted. The U.S. in 1960 suspended sugar imports from Cuba, dissolved ties to the Castro government, and halted all Cuban assets at home. In March of 1960, President Dwight D. Eisenhower directed the CIA to start the training of Cuban exiles to overthrow Castro and allocated $13.1 million to the cause.

Bay of Pigs
1961: Members of Castro's militia in the Escambry Mountain area of Cuba during the ill-fated US backed Bay of Pigs invasion. Three Lions/Getty

1961: Bay of Pigs

- Army strength: 858,622 people
- Navy strength: 626,223 people
- Marine Corps strength: 176,909 people
- Air Force strength: 821,151 people
- Total strength: 2,482,905 people
- Percent of population enlisted: 1.35%

Though the threat of nuclear war remained low, newly appointed President John F. Kennedy nevertheless vowed to equip the U.S. military with the necessary means to retaliate if attacked. Kennedy warned the Soviet Union against getting involved in the U.N. establishment of peace and independence in the Congo. The Bay of Pigs invasion took place in Cuba on April 17, 1961, sending almost 1,500 trained Cuban exiles storming the Bay of Pigs beaches. The initial air strike on Cuba's airfield didn't dismantle the entire air force, however, ultimately causing a failed attempt to overthrow the growing Communist government.

1962: Cuban Missile Crisis
/AFP / Getty Images

1962: Cuban Missile Crisis

- Army strength: 1,066,404 people
- Navy strength: 664,212 people
- Marine Corps strength: 190,962 people
- Air Force strength: 884,025 people
- Total strength: 2,805,603 people
- Percent of population enlisted: 1.50%

When it was revealed that Cuba was in possession of Soviet ballistic missiles, the U.S. government set up a naval blockade in southern Florida. This 13-day incident became known as the Cuban Missile Crisis. President John Fitzgerald Kennedy warned the Soviet Union that if they launched missiles from Cuba against any country, the U.S would be forced to use nuclear weapons on Russia. The issue was resolved when Soviet leader Nikita Khrushchev called for the removal of the Soviet missiles from Cuba.

1963: Limited Nuclear Test Ban Treaty

- Army strength: 975,916 people
- Navy strength: 663,897 people
- Marine Corps strength: 189,683 people
- Air Force strength: 869,431 people
- Total strength: 2,698,927 people
- Percent of population enlisted: 1.43%

Representatives from the Soviet Union, Great Britain, and the United States on Oct. 7, 1963, signed the Limited Nuclear Test Ban Treaty. The document effectively banned nuclear testing in space, underwater, and in the atmosphere, but did allow for testing in underground sites with responsible containment of radioactive material.

1964: Gulf of Tonkin

- Army strength: 973,238 people
- Navy strength: 665,969 people
- Marine Corps strength: 189,777 people
- Air Force strength: 856,798 people
- Total strength: 2,685,782 people
- Percent of population enlisted: 1.40%

As Saigon's government continued to decline, President Lyndon B. Johnson sent military advisers to South Vietnam to assist. U.S. destroyers in the Tonkin Gulf in August 1964 radioed that they were receiving fire by North Vietnamese forces, inspiring President Johnson to request increased military force in Indochina. Congress drew up the Tonkin Gulf Resolution, which gave the green light for any means necessary to stave off attacks and halt further violence and ultimately managed to intensify the conflict in Vietnam.

1965: U.S. officially enters Vietnam War

- Army strength: 969,066 people
- Navy strength: 669,985 people
- Marine Corps strength: 190,213 people
- Air Force strength: 824,662 people
- Total strength: 2,653,926 people
- Percent of population enlisted: 1.37%

As the 9th Marine Expeditionary Brigade arrived on the shores of China Beach on March 8, 1965, the U.S. officially entered the Vietnam War. Prior activity in Vietnam—including 23,000 military advisers already there and various naval action—did not include active fighting and was being used largely for support and intelligence. Throughout the Vietnam Era, more than 2,709,918 Americans—9.7% of their generation—served in Vietnam.

1966: House Un-American Activities Committee

- Army strength: 1,199,784 people
- Navy strength: 743,322 people
- Marine Corps strength: 261,716 people
- Air Force strength: 887,353 people
- Total strength: 3,092,175 people
- Percent of population enlisted: 1.57%

The House Un-American Activities Committee (HUAC) was an investigative committee within the U.S. House of Representatives that was founded in 1938 to investigate potential disloyal American citizens. HUAC in 1966 began hearings to investigate citizens alleged to be helping the Viet Cong as well as anti-war activists. Hundreds of protesters showed up to the hearings on the first day, Aug. 16, 1966, while witnesses resisted their questioners. The hearings and protests incited more anti-Vietnam War demonstrations around the country.

1967: Operation Swift

- Army strength: 1,442,498 people
- Navy strength: 750,224 people
- Marine Corps strength: 285,269 people
- Air Force strength: 897,494 people
- Total strength: 3,375,485 people
- Percent of population enlisted: 1.70%

U.S. Marines carried out Operation Swift beginning Sept. 4, 1967, in order to free two Marine companies that had been waylaid by the Peoples Army of Vietnam. The rescue mission, involving three 5th Marine Regiment battalions up against bigger forces of NVA and Viet Cong, took place in the Quang Nam and Quang Tin provinces and resulted in two Medal of Honor recipients, the saving of the city of Da Nang, and an estimated enemy casualty count surpassing 4,000.

1968: Tet Offensive

- Army strength: 1,570,343 people
- Navy strength: 763,626 people
- Marine Corps strength: 307,252 people
- Air Force strength: 904,850 people
- Total strength: 3,546,071 people
- Percent of population enlisted: 1.77%

A coordinated 1968 attack during the Tet holiday by North Vietnamese and communist Viet Cong forces (called the Tet Offensive) resulted in extensive casualties. The attack came at a time when the United States was claiming the war was all but won, ensuring a further dropout of support for the Vietnam War by the American public.

1969: Nixon Doctrine

- Army strength: 1,512,169 people
- Navy strength: 773,779 people
- Marine Corps strength: 309,771 people
- Air Force strength: 862,353 people
- Total strength: 3,458,072 people
- Percent of population enlisted: 1.71%

Newly appointed President Richard Nixon established the Nixon Doctrine (first called the Guam Doctrine) in 1969. This document established that the U.S. would rely on its Asian allies to take control of their own military defenses, while still receiving some support from their American neighbors.

1970: U.S. troops invade Cambodia

- Army strength: 1,322,548 people
- Navy strength: 691,126 people
- Marine Corps strength: 259,737 people
- Air Force strength: 791,349 people
- Total strength: 3,064,760 people
- Percent of population enlisted: 1.49%

As the Vietnam War began to wind down, U.S. troops invaded Cambodia in 1970 to capture lingering Viet Cong forces and prevent North Vietnamese attacks on South Vietnam. On April 30 President Nixon gave a speech explaining his decision, which touched off a fresh set of anti-war protests that resulted in the notorious Kent State shootings where four protestors were killed and nine injured.

1971: Conviction in My Lai Massacre

- Army strength: 1,123,810 people
- Navy strength: 621,565 people
- Marine Corps strength: 212,369 people
- Air Force strength: 755,300 people
- Total strength: 2,713,044 people
- Percent of population enlisted: 1.31%

Army Lieut. William Calley was convicted of the 1968 deaths of 22 civilians during the My Lai Massacre, in which more than 500 people were killed. He was sentenced to life in prison, but President Richard Nixon reduced his sentence and Calley ended up serving just three years under house arrest.

1972: Nguyen Hue Offensive

- Army strength: 810,960 people
- Navy strength: 586,923 people
- Marine Corps strength: 198,238 people
- Air Force strength: 725,838 people
- Total strength: 2,321,959 people
- Percent of population enlisted: 1.11%

The North Vietnamese launched the Easter Offensive (dubbed the Nguyen Hue Offensive) on March 30, 1972, which was a large-scale, three-part assault on South Vietnam. Hundreds of South Vietnamese civilians and soldiers were injured as a result, but the attack was ultimately stopped in October of that year by South Vietnamese soldiers and U.S. advisers.

1973: Ceasefire signed

- Army strength: 800,973 people
- Navy strength: 563,683 people
- Marine Corps strength: 196,098 people
- Air Force strength: 691,182 people
- Total strength: 2,251,936 people
- Percent of population enlisted: 1.06%

President Nixon announced progress in peace negotiations with Vietnam, and called for the cessation of offensive action in North Vietnam. A ceasefire was signed, but soon violated by the communists in March of 1973. All-out war had resumed by 1974.

Richard Nixon
4/30/1970-Washington, DC-In a TV speech to the Nation from the White House, President Nixon announced that several thousand American ground troops have entered Cambodia to wipe out Communist headquarters for all military operations against South Vietnam. The President is shown here standing before a map of Cambodia. Getty

1974: U.S. evacuation of Cyprus

- Army strength: 783,330 people
- Navy strength: 545,903 people
- Marine Corps strength: 188,802 people
- Air Force strength: 643,970 people
- Total strength: 2,162,005 people
- Percent of population enlisted: 1.01%