"Yesterday I let them know that I was extremely unhappy with what was happening", Trump said at an impromptu press conference, adding, "Now we are very happy".
"Tremendous progress has been made, everyone's agreed to substantially up their commitment, they're going to up it at levels they've never thought of before", Trump told a press conference before leaving for a visit to Britain.
Before leaving on Thursday for a visit to the United Kingdom, Trump called USA commitment to the alliance "very strong", and referred to the two-day summit as "fantastic" characterized by "a great collegial spirit". The U.S. military budget comprises about 70 percent of the military spending of all North Atlantic Treaty Organisation countries together, not 90 percent - and that's for worldwide military commitments, not just Europe.
He has also crossed swords with French President Emmanuel Macron and Canadian Prime Minister Justin Trudeau, America's biggest trading partner.
"Some are at 2%, others have agreed definitely to go to 2%, and some are going back to get the approval, and which they will get, to go to 2%", he said.
At present, not even the United States spends 4% of GDP on defense with only five - USA, UK, Estonia, Poland, and Greece - out of NATO's 29 members now meeting the agreed defense spending target of 2% GDP set for countries to meet by 2024 in accordance with a 2014 agreement.
He earlier infuriated Chancellor Angela Merkel by claiming Germany had been left "captive" and was "totally controlled" by Moscow because of a gas pipeline deal with Russian Federation.
During a meeting with Georgia and Ukraine - two nations that hope to join NATO - Trump turned conversation to his spending concerns, which caused the secretary general to call an emergency budget meeting, according to The New York Times.
The issue dominated the president's public remarks after he arrived in Brussels on Tuesday, striking fear among European officials that the president would question the US' role in the alliance.
Politico said Trump warned of "grave consequences" if the allies did not immediately meet the 2 per cent spending targets.
That mercurial side of Trump is less likely to be experienced Monday by Russian President Vladimir Putin, with whom Trump has been anticipating a sit-down for weeks.
Trump did not directly answer questions about whether he had threatened to take the U.S. out of the alliance, saying he "told people I'd be very unhappy if they didn't up their commitments very substantially".
"We had a very intense summit", said Merkel.
In August of 2016, at the height of the United States presidential election race, 50 Republican national security experts warned then Republican nominee Donald Trump would be "the most reckless president" in American history.
They've reaffirmed their commitment to Afghanistan and promised support through 2024.
As well as Merkel, Spanish Prime Minister Pedro Sanchez and Charles Michel, the prime minister of Belgium, were also singled out by Trump for undershooting on their spending targets when United States taxpayers, funding a defence budget worth about 3.6% of their national income, foot much of Nato's bills.
Two of her top government officials resigned earlier this week over negotiations on how to manage Brexit, including foreign secretary Boris Johnson, whom Trump has called a "friend".
The alliance, which sees an attack on one member as an attack on all, was set up at the height of the Cold War, as a counterbalance to the Warsaw Pact between the Soviets and their satellite states.
"I believe through strength you get peace", he said.
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