Tensions rose on the final day of the two-day summit, when members met in an emergency session amid demands from Trump to speed up defense spending.
As a contentious North Atlantic Treaty Organization summit in Brussels began to wind down on Thursday, President Donald Trump claimed victory over defense spending and reaffirmed US commitment to the alliance. "Since President Trump was in Brussels last spring allies have added 41 billion extra for defence spending".
Mr Trump said he told counterparts on Wednesday that he was "extremely unhappy" with the lack of progress being made towards meeting the alliance target of spending 2% of GDP on defence. But there was little evidence other leaders had bowed to his wishes on that front.
Trump suggests he forced a great shift in the space of the past two days.
"We consider that it's a question of economic competition and that ... the buyers must take their own decision", he said.
Trump and first lady Melania Trump landed at Stansted Airport, 40 miles northeast of the British capital Thursday afternoon local time. The U.S. share of that is capped at 22 percent.
Trump had opened the first day of talks on 11 July with a public diatribe against Germany, the second biggest state in the Western defence alliance, criticising its reliance on Russian gas imports and failure to spend more on defence.
Most nations are reluctant to commit major portions of their GDP annually to buying these pricey United States arms, however. He wanted 4 percent.
The Russian ministry's spokeswoman, Maria Zakharova, defended the USA president's comments, saying on Facebook that Trump had "revealed his North Atlantic Treaty Organisation strategy long before the [presidential] election", sharing his thoughts on the alliance being "obsolete" and too costly during his campaign and before becoming a politician.
He also said meeting the queen would be an experience Trump "will really cherish".
And the president's declaration was also directly contradicted by one of his closest personal allies, French President Emanuel Macron.
Thursday, Trump had said, "I think they like me a lot in the U.K".
"So that was on Barack Obama's watch".
In what sounded like a Johnson endorsement, Mr. Trump went on to say, "Boris Johnson is a friend of mine".
GREENE: Ayesha, let me start with you.
Mr Trump's early-morning tweets from Brussels had been typically critical.
When Canadian Prime Minister Justin Trudeau said Canada would not be pushed around by the United States on trade, Trump responded with a tweet from Air Force One calling Trudeau "very dishonest and weak".
"I'm a very stable genius", he said.
Stoltenberg, stepping in - not for the first time - as peacemaker between Trump and the other 28, called a special closed-door session, the first in a decade, with most officials and the invited guests ushered out, to allow the alliance's principal leaders to remonstrate with Trump.
Conservative writer Shermichael Singleton defended President TrumpDonald John TrumpGiuliani: Trump interview with Mueller "further away" ACLU calls for Trump officials to hand over info on immigrant children Kushner to join Pompeo for meetings with Mexican leaders MORE's understanding of global alliances on Friday after the president's tumultuous North Atlantic Treaty Organization (NATO) summit this week. "They pay only a fraction of their cost".
"On top of it all, Germany just started paying Russian Federation, the country they want protection from, Billions of Dollars for their Energy needs coming out of a new pipeline from Russian Federation".