Trudeau called the military spending target - two per cent of GDP, agreed to by all North Atlantic Treaty Organisation allies at the 2014 summit in Wales - "an easy shorthand" but also a "limited tool" to measure a country's commitment to the alliance.
Prime Minister May said Trump is well-positioned as he prepares to meet with Putin, saying "what is important is that the president goes into this as he is doing from a position of strength and also from a position of unity in North Atlantic Treaty Organisation". That's according to Germany's Süddeutsche Zeitung.
French President Emmanuel Macron quickly disputed Trump's claim that North Atlantic Treaty Organisation allies had agreed to boost defence spending beyond the two per cent benchmark, citing the group's communique that lays out the original goal.
It is not clear whether he was talking about a partial or full withdrawal from North Atlantic Treaty Organisation, or whether he was serious about the threat.
Trump wants other North Atlantic Treaty Organisation member states to increase defense spending - and soon. They will then hold a working lunch with their full delegations.
"There was a commitment to reverse that declining investment and start moving toward two per cent and that's exactly what we agreed with as a government when we came in".
"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".
Following Thursday's emergency session of North Atlantic Treaty Organisation members, Trump declared the military alliance to be "very unified, very strong, no problem".
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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 US taxpayers, funding a defense budget worth about 3.6 percent of their national income, foot much of NATO's bills.
At the moment, North Atlantic Treaty Organisation members are supposed to spend 2% of GDP on defense-a goal, not a requirement or a membership fee-but only eight of the alliance's 29 members will do so this year.
However, the US President suggested he wanted the target to rise to 4% of GDP. "The U.S. pays tens of Billions of Dollars too much to subsidize Europe, and loses Big on Trade!"
"Trump has maybe a little less room to maneuver if he wants to downplay the issue or pretend that it's not real", said Jeffrey Mankoff, a Russian Federation expert at the Washington-based Center for Strategic and International Studies.
North Atlantic Treaty Organisation is an alliance that was formed in the wake of World War II as the United States and its allies sought to counter the Soviet Union's growing influence in Europe and beyond.
Trump added that although he did not know Putin very well, he hoped that one day he might become friends with him.
He said the mission, to begin in the fall of 2018 in Baghdad and the surrounding vicinity, will include the deployment of as many as 250 Canadian Armed Forces personnel and up to four Griffon helicopters to support North Atlantic Treaty Organisation activities.
This article was updated as more details emerged.