The leaders, including Trump, had signed on to a statement that highlighted agreement on a plan to improve the readiness and mobility of the armed forces across the Continent, and they agreed on tough language aimed at Moscow. He said they spent about 90 minutes talking at dinner and claimed they "probably never developed a better relationship than last night".
Still, European economies have been growing recently, making the 2 percent of GDP target ever harder to achieve, and Trump's spending demand is at least in part aimed at drumming up business for the USA defense industry.
"I told people that I'd be very unhappy if they did not up their commitments very substantially", Trump told reporters.
Before leaving for the Nato Summit in Brussels Tuesday morning, Trump tweeted, "The U.S.is spending many times more than any other country in order to protect them". "We committed ourselves to spending a bit more".
Earlier, Trump strode into NATO's new headquarters to shake hands with leaders - but only after publicly railing against Germany at a breakfast meeting with Stoltenberg.
Then he claimed "total credit" for increases in defence spending, although they were in fact agreed in 2014, and warned that "NATO is helping Europe more than it is helping us", in language that did not make a public case for unity.
Asked if he could accept Russia's annexation with Crimea and still get along with Putin, he said, 'When it comes to Crimea, there's nothing much I have to say about it other than we will look at that just like I'm looking at many disasters that I've taken over'.
"But ultimately he is a competitor".
Although Trump administration officials point to the long-standing alliance between the United States and the United Kingdom, Trump's itinerary in England will largely keep him out of central London, where significant protests are expected. "Hopefully some day he will be a friend". So, I mean, could both Macron and Trump be correct, and they're just sort of emphasizing different sides of this whole thing?
But just hours after the president boarded Air Force One to leave the Brussels summit, USA officials at the Pentagon were reportedly on full-fledged "damage control", calling foreign leaders to assure them the US will continue to provide military support for North Atlantic Treaty Organisation, according to NBC News.
Trump had spent his time in Brussels berating members of the military alliance for failing to spend enough of their money on defence, accusing Europe of freeloading off the US and raising doubts about whether he would come to members' defence if they were attacked.
"I believe through strength you get peace", he said. "He may deny (it)", Mr Trump added.
Trump is in London, where his day will start with a viewing of a military demonstration before he heads to meetings with British Prime Minister Theresa May at Chequers, May's county house. Mark Warner, the top Democrat on the Senate Intelligence Committee, said he is concerned that the President's "ad-hoc" style of going into meetings and "winging it" is not appropriate and that the meeting should be called off with Putin.
On spending, he said "we are stepping up as never before, allowing U.S. spending to go down", and there was a "new sense of urgency due to president Trump's strong leadership on defence spending".
And the president's declaration was also directly contradicted by one of his closest personal allies, French President Emanuel Macron.
The president will also be paying a visit to Windsor Castle where he and first lady Melania Trump will have tea with Queen Elizabeth II.
He reportedly continued by warning that if the alliance's wealthiest economies weren't paying 2% by January, then the United States "are going to do our own thing".