Sources familiar with the first round of closed-door talks on Thursday told the Reuters news agency that Mr. Trump carried his firm stance on military spending from Twitter into the meeting rooms.
Trump said he wanted "peace all over the world, that's my goal", and he wanted to achieve this by building up the military, with his dream to have the best military equipment in the world and never have to use it.
"We're supposed to be guarding against Russian Federation and Germany goes out and pays billions and billions of dollars a year to Russian Federation".
As a result, he continued, "NATO is much stronger than it was two days ago".
Trump said his summit with Putin, which will take place on Monday in Helsinki, would be a loose meeting with "no big schedule" - but one that could still be productive.
"Germany is totally controlled by Russian Federation", the president said.
Leaders of Azerbaijan and Georgia, non-NATO members, were asked to leave for the emergency session. In the afternoon, he heads to his next stop: the United Kingdom.
Though NATO allies have squabbled in the past, Trump seemed to be leaving in his wake not just bruised feelings, but deeper concerns about the future of the Western alliance that has served as a bedrock of the worldwide order. "It's really extraordinary that Donald Trump has to be the person to point out that the emperor in Europe has no clothes".
The comments come amid wide expectations of a turbulent summit-Trump has long criticized the 29-member alliance, calling it "obsolete", and is extremely critical of countries that are failing to meet the target of 2% of their GDP as defense spending.
The president has been highly critical of the alliance, complaining the United States pays more than others.
Trump has described the spending situation as "disproportionate and not fair to the taxpayers of the United States".
But White House Chief of Staff John Kelly may have stolen the show at the breakfast, with his eye rolling and fidgeting as Trump was attacking Germany. "I'm a very stable genius".
"I think we will be able to", he said.
Speaking to a cheering crowd at a rally this week, Trump told them that the United States would no longer be "the schmucks paying for the whole thing".
It was Trudeau's second bilateral meeting with a North Atlantic Treaty Organisation counterpart following a sitdown Wednesday with Swedish Prime Minister Stefan Löfven.
The U.S. Embassy has warned Americans to be careful in London during Trump's visit, recommending that they "exercise caution if unexpectedly in the vicinity of large gatherings that may become violent", "keep a low profile" and "be aware of your surroundings". And one of the freedoms we have is freedom of speech and the freedom to express your views. "I'm very consistent. I'm a very stable genius".
Officials at the meeting said Trump broke with diplomatic protocol by addressing German Chancellor Angela Merkel by her first name, telling her: "Angela, you need to do something about this".