NATO leaders recently said they are "further strengthening our ability to deter and defend with an appropriate mix of nuclear, conventional, and missile defense capabilities."
The president told supporters in Hershey that the country was "laughed at" four years ago under the Obama administration.
The president previously described a construction project in Istanbul as "a little conflict of interest" in his dealings with Turkey.
Chris Van Hollen said Turkish President Recep Tayyip Erdogan is being allowed to cozy up to Putin "with impunity."
Sen. Elizabeth Warren said three years of the Trump presidency has put the country's alliances under "enormous strain."
NATO defense spending has jumped over $100 billion a year from 2016 to 2019.
Trump spent his two days in London lashing out at allies and creating new rifts, but host Boris Johnson avoided the worst of the flak by limiting his interaction with the president.
The campaign ad claimed the world saw Trump as "dangerously incompetent" at the summit.
Trump canceled a planned press conference after world leaders were filmed joking together at the NATO summit, apparently about the president, although his name was not mentioned.
In a clip from a NATO reception that went viral on Wednesday, world leaders including Boris Johnson appeared to make fun of the U.S. president.
Pete Souza took aim at the president after his early departure from the 70th anniversary NATO summit following a spat with Justin Trudeau.
Fox News hosts quickly came to the defense of "dominant" President Donald Trump after a video emerged of world leaders apparently mocking the president at a NATO reception.
The reason Giuliani must have made the calls was "not so complicated," the president said Wednesday.
While some saw the viral video as a gossiping session between world leaders, Trudeau said he was "pleased" to share the story behind the announcement.
"These are people who run countries...these are people who are talking about serious things," Tom Nicols said in an interview with MSNBC's Joe Scarborough. "And the president is always out of his depth in those environments."
The husband of Trump counselor Kellyanne Conway claimed the video showed that the world thinks the president is a "deranged buffoon."
Trump cancelled a press conference and left the meeting earlier amid a spat with Justine Trudeau, but Jens Stoltenberg said unity within the alliance was "iron-clad."
Trump told reporters he had done enough press conferences and that he would "probably" leave the London meeting early.
The president also said the Canadian Prime Minister was a "very nice guy" after a long pause following the insult.
The president attacked the House Intelligence Committee chairman during a meeting with Canadian Prime Minister Justin Trudeau on Tuesday.
Queen Elizabeth II's only daughter was spotted with Justin Trudeau, Emmanuel Macron and Boris Johnson at a Buckingham Palace reception Tuesday.
Turkey's invasion of northeastern Syria has prompted protests from other NATO nations and opened up cracks in the alliance.
Canada is among the NATO nations spending less than 2 percent of GDP on military investments, much to Trump's frustration.
One organizer told Newsweek that the Baby Trump blimp could still be in America following a Florida rally.
Jens Stoltenberg said he remains a believer in a dual approach of deterrence and dialogue towards Russia.
During Tuesday's 'Morning Joe,' host Joe Scarborough mocked Donald Trump's claims that he had legal scholars read the transcripts of his Ukraine phone calls.
Donald Trump heads to a reception for NATO leaders at Buckingham Palace, but Prince Andrew will not be in attendance.
The president is visiting the U.K. for the NATO summit in London from December 3-4.
Prime Minister Boris Johnson's team is bracing for a possible intervention from Trump, who is widely unpopular in the U.K.