As he tried to reach the front of the group, Montenegro's Prime Minister Dusko Markovic was standing in his way.
After months of criticizing NATO, President Donald Trump is attending his first NATO summit.
The act was being widely denounced online.
"Our NATO allies responded swiftly and decisively", Trump said, "invoking for the first time in its history the Article 5 collective defense commitments". "These grave security concerns are the same reason that I have been very, very direct with the secretary and members of the alliance in saying that nation members must finally contribute their fair share and meet their financial obligations".
The moment followed Trump's speech at the headquarters' opening, during which he slammed USA allies for not spending enough on defence before an uncomfortable-looking audience of fellow heads of state.
"Many of these nations owe massive amounts of money from past years", Trump said yesterday in Brussels.
Diplomatic sources said some member states such as France, Germany and Italy had opposed such a move for fear the alliance would be dragged into a ground war and risk relations with Arab powers.
According to White House press secretary Sean Spicer, it was a very positive reaction and affirmation of Trump's priorities.
The North Atlantic Treaty Organization (NATO) would be a full member of the global coalition against the Islamic State (IS), NATO chief said.
Ms Merkel, who stood emotionless during the speech, earlier issued her own challenge to the U.S. president, saying that member states including Germany had already pledged to work toward the 2 per cent target by 2024 and that outlays on security more generally also count toward the goal.
The E.U.'s two leaders who share the job of running the bloc of 28 countries, the president of the European commission, Jean-Claude Juncker, and the president of the European Council, Donald Tusk, also met President Trump today.
"Some issues remain open, like climate and trade".
Some NATO leaders, including Prime Minister Xavier Bettel of Luxembourg and President Emmanuel Macron, were reportedly seen sharing secret side-glances with amused looks in their eyes as Trump lectured sternly from the podium.
Some fractures between the USA and its allies are visible on Mr. Trump's final stop, in sharp contrast to the warm reception he got in the Mideast, reports CBS News' Margaret Brennan.