At a news conference wrapping up the two-day North Atlantic Treaty Organisation summit in Brussels, Trudeau was pressed to provide more details about the US president's sudden insistence that allies have agreed to spend more - and to do it more quickly.
US officials told NBC News that maintaining "predictability" among alliance members was important and that calls were made with foreign officials centered around "reinforcing alliance commitments" after Trump "made it clear alliance commitments were on the table".
U.S. President Donald Trump holds a news conference after participating in the NATO Summit in Brussels, Belgium July 12, 2018. "Everybody in that room got along and they agreed to pay more and they agreed to pay it more quickly".
"So if you go back 10 or 20 years, you'll just add it all up". "NATO", he added, "is much stronger now than it was two days ago".
At one point, Emmanuel Macron had to pause his speech for nearly a minute, waiting for Trump to finish his private conversation and refocus on the meeting.
"What they're doing is spending - at a much faster clip they're going up to the 2 percent level", Trump said.
"Yesterday I let them know that I was extremely unhappy with what was happening and they have substantially upped their commitment".
Politico said Trump warned of "grave consequences" if the allies did not immediately meet the 2 per cent spending targets. "I'm very consistent. I'm a very stable genius".
Before he visits with Putin, Trump will visit London and the United Kingdom - where many large protests are planned against his policies.
Bringing chaos with him as he moves across Europe, U.S. President Donald Trump's pomp-filled welcome to Britain was overshadowed Friday by an explosive interview in which he blasted Prime Minister Theresa May, blamed London's mayor for terror attacks against the city and argued that Europe was "losing its culture" because of immigration.
The second day of the defence pact meeting was disrupted and repeatedly rescheduled, as Trump's insistence on renegotiating his allies' defence budgets took precedence over issues such as Ukraine and Georgia's vulnerability to Russian Federation.
It's also always been a bugbear for the United States, which spends more on defence both as a percentage of GDP and in absolute terms than any other member. Some NATO nations, like Turkey, have even chosen to buy Russian over U.S. arms in recent years, a fact that doesn't sit well with the administration.
The reason is that the 2 percent standard came from a collective defense agreement against a formidable foe (the Soviet Union) that doesn't exist anymore.
In a series of tweets at the time, Trump said that "throughout my life, my two greatest assets have been mental stability and being, like, really smart".
But just days out from a high-stakes meeting with Russian President Vladimir Putin in Helsinki, the mercurial USA leader threatened to throw the transatlantic alliance into disarray.
Demonstrators say that hundreds of thousands of people are planning to take to the streets tonight and tomorrow, and are planning to float a giant balloon caricature of Trump outside Parliament.
"I believe through strength you get peace", he said.