It came while defending himself against allegations he was unfit to be USA president following publication of the book, Fire And Fury: Inside The Trump White House.
Before arriving a half hour late to the final day of meetings on Thursday, Trump tweeted that "the US pays tens of Billions of Dollars too much to subsidize Europe", that Germany's oil and gas pipeline with Russian Federation is "not acceptable", and that allies must not only all reach the 2 percent defense spending guideline, but double that to 4 percent.
He also said meeting the queen would be an experience Trump "will really cherish".
"(The) 2 per cent (figure) was a range, a goal, now it's a commitment, it was like this amorphous number out there and now it's a real commitment", he said. "And that's fine. Of course, they kill us on trade".
In a tweet today after a day of meetings, in which he lashed out at allies for insufficient defense spending, Trump calls on member nations to immediately boost spending.
"Prior to a year ago, where I attended my last meeting it was going down, the amount of money being spent by countries was going down and down very substantially", he said.
Trump did not directly answer questions about whether he had threatened to take the USA out of the alliance, saying he "told people I'd be very unhappy if they didn't up their commitments very substantially".
"Yesterday I let them know that I was extremely unhappy with what was happening", he said.
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At 11am in Brussels when the first reports started dropping that Donald Trump had let loose, again, on his North Atlantic Treaty Organisation allies - this time with a possible threat added that the U.S. would simply pull out of the alliance - it didn't take long to spread among the press corps. He urged allies on Wednesday to double their defense spending targets to 4 percent of GDP.
In particular, Trump did not meet with Romanian President Klaus Johannis.
Nothing has changed since Trump arrived in Brussels.
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.
The same is true for New York Jets owner and GOP megadonor Woody Johnson, the USA ambassador to the United Kingdom; and former Utah Gov. Jon Huntsman, the US ambassador to Russian Federation.
Merkel, who grew up in communist East Germany, shot back that she knew what it meant to be under Kremlin domination and Germany had the right to make its own policy choices. He described the U.K.as a "hot-spot with lots of resignations".
One idea Trump floated last week was the threat of a drawdown of the 35,000 USA troops in Germany. On Thursday, Trump said he wasn't troubled by the planned protests. "I think that would qualify as not smart, but genius...and a very stable genius at that!"
"So I would imagine that's what they will do, but maybe they will take a little bit of a different route".