The West is facing security threats unlike any since the end of the Cold War, but a dearth of leadership continues to plague the alliance.
Advent starts this week as the Judiciary Committee begins its impeachment hearings.
The U.S. will cut its contribution to the direct NATO budget, which covers maintenance of the NATO headquarters, joint security investments and some combined military operations.
The president has publicly antagonized America's European Union allies, even describing the bloc as a "foe."
Turkey began radar testing of its Russian-made S-400 anti-aircraft system on Monday, the purchase of which has created friction with Washington.
In the 70 years since its founding, the U.S.-led NATO military alliance has steadily expanded into Eastern Europe, bringing with it new weapons viewed as provocative by Russia.
The president has repeatedly maligned the Cold War-era alliance, even suggesting the pact is obsolete.
To keep Russia and Iran in check, NATO now more than ever needs to develop and pursue a comprehensive strategy for the Black Sea. Georgia is key to the region.
Former U.S. Navy intelligence and counter-terrorism specialist Malcolm Nance suggested, "Putin leads Donald Trump around on a leash."
Trump's National Security Adviser Robert O'Brien said Germany and other NATO members "not paying their fair share" of defense spending is breaking up the alliance.
Macron warned this week that the NATO alliance was experiencing "brain death" as a result of erratic U.S. foreign policy.
"They're managing currently to conduct global operations without owning a shred of territory," retired four-star Admiral James Stavridis said
"We should in a bipartisan way seek to ... change Turkey's behavior," Democratic Representative Eric Swalwell said.
Some 50 U.S. warheads are believed to be deployed at the Incirlik air base in southern Turkey, but reports this weekend suggested officials were reviewing plans for their removal.
Turkey is facing widespread condemnation for its military operation against Kurdish-led forces in northeastern Syria.
"We are voluntarily abdicating our role and it's the Russians, Iranians and others who are benefitting," Richard Haass warned.
General Joseph Dunford said that the new NATO strategy adopted in May was part of the alliance's response to the growing Russian threat.
Anders Fogh Rasmussen told Newsweek that despite Trump's public condemnation, increased U.S. activity has helped the alliance become stronger than it was at the start of the president's term.
Relations between NATO allies the U.S. and Turkey have been strained by Ankara's decision to purchase the Russian-made S-400 anti-aircraft system.
Foreign Minister Sergei Lavrov also warned that proposed NATO expansion poses a threat to peace in Europe.
There are around 150 U.S. nuclear weapons stored in six European countries in what was an open secret but never before publicly discussed.
Turkey's purchase of the Russian anti-aircraft system has opened up a rift with the U.S., which fears the deal could compromise American military secrets.
Russia has long expressed concern that its neighbors could become NATO members and that such developments could spark war.
Ukraine's government has long aspired to join NATO.
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NATO could gain a 30th member by as early as October.
A Fox News panel expressed fears President Donald Trump may inflame "leave" Brexit and anti-NATO sentiment during his U.K. visit.
Despite Norwegian and U.S. denials, the Globus-II radar previously served in a role related to missile technology and was once revealed to be pointing toward Russia.
Washington's demand that Turkey kill a deal for Russia's S-400 anti-aircraft system was "unacceptable," a Kremlin spokesman said.