Striking a strident tone at the summit, Trump's aspirational target of 4 percent of gross domestic product (GDP) was above the United States own spending on defense.
US officials have told Reuters that Washington is preparing another review of strategy, a year after Trump begrudgingly agreed to extend involvement in the 17-year-old war.
The document issued at the North Atlantic Treaty Organisation leaders' summit reads: "We reaffirm our unwavering commitment to all aspects of the Defence Investment Pledge agreed at the 2014 Wales Summit, and to submit credible national plans on its implementation, including the spending guidelines for 2024, planned capabilities, and contributions".
"President Trump wants to see our allies share more of the burden and at a very minimum meet their already stated obligations", she said. "Why are there only 5 out of 29 countries that have met their commitment?"
Despite Trump's pugnacious posture and rhetoric, allies sought to project unity at the conclusion of meetings in Brussels. "The good news is that we are making progress". Russian Federation needs us to help with their economy, something that would be very easy to do, and we need all nations to work together.
Perhaps that's why, despite his outbursts, Mr. Trump did not stand in the way of NATO's adoption at the summit of new plans to defend against Russian Federation and terrorism.
"We will continue to stand together and act together, on the basis of solidarity, shared objective, and fair burden-sharing", it said.
Earlier in the day, after Trump blasted Germany for buying Russian oil and natural gas, NATO Secretary General Jens Stoltenberg sought to convey an attitude of slow, steady progress for the alliance. The U.S.is paying for Europe's protection, then loses billions on Trade.
"Germany is a rich country, they talk about increasing it a tiny bit by 2030".
He said that German Chancellor Angela Merkel runs the nation that is the "de facto leader of the EU" and that its failure to "properly fund NATO" should be criticized. "Because of that we can say that we can make our independent policies and make independent decisions".
Trump didn't identify Nord Stream 2 by name. Allies will deliver on fairer burden-sharing, he said, pointing out that European Allies and Canada are expected to spend an extra $266 billion on defence between now and 2024.
"It was also because of your leadership", Stoltenberg told Trump.
President Trump is set to meet Russian President Vladimir Putin in Helsinki on July 16.
Alluding to Trump's comments about Germany, Kerry also said that "European energy security is an important topic for allies to discuss, even when they disagree". "And it can't be explained, and you know that".
Republicans and Democrats alike thumbs-downed the tariffs list that dropped Tuesday, as did US business owners who expect to be hurt by them.
Trump's laser-focus on Germany has unsettled Berlin, which had grown accustomed to a strong relationship with President Barack Obama.
"I am pleased to have this opportunity to be here for this exchange of views", she said, through a translator.
Journalists were allowed briefly into the room at the end of their meeting, but both leaders avoided publicly blasting each other face-to-face.