The failure of many allies to release a plan to achieve the two percent goal has drawn the fury of the president, even as North Atlantic Treaty Organisation leaders acknowledge they need to be more aggressive in modernizing their military capabilities.
Writing on Twitter, President Trump said: "Why are there only five 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. "Because of that we can say that we can make our independent policies and make independent decisions". "That makes America less safe".
When the Brussels summit winds up, Mr Trump will arrive in the United Kingdom on Thursday for a two-day working visit.
Mr Trump dominated the day and is set to stay in the spotlight as he visits the United Kingdom and prepares for a summit with Russian president Vladimir Putin.
As the day began, Trump wasted no time going on the offensive as he began a week of high-stakes diplomacy on both sides of the former Cold War divide.
"We should first get to two percent", NATO Secretary-General Jens Stoltenberg said, adding that eight of the 29 allies were meeting that target, while others had a plan to do so - turning a leaf on years of defence budget cuts. "We're supposed to protect you against Russian Federation but they're paying billions of dollars to Russian Federation and I think that's very inappropriate". His comment that Germany was controlled by Russian Federation earned a rebuke from Berlin.
Under NATO rules, countries with territorial conflicts can not join the Western alliance and neither country is expected to progress in membership talks.
Dr. Claudia Major, a researcher at the German Institute for International Security affairs, said that Germany, in particular, does more for the alliance than spend on its own defense, including deploying troops to Afghanistan, "and eventually, two percent is an arbitrary measurement that does not say much about efficiency and output".
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We are excited about the future but also we want to make the most of the opportunity we have tomorrow'. It's up to us now which games we play in".
Earlier, Trump strode into NATO's new headquarters to shake hands with leaders - but only after publicly railing against Germany at a breakfast meeting with Stoltenberg.
Trump has preferred to take aim at allies. "This, not defense spending or pipelines, is the new fault line in the alliance today and the greatest in its history".
Trump has long complained that European NATO members do not pay enough for their own defence, accusing them of freeloading on America and singling out Germany for particular criticism.
"It was also because of your leadership", Stoltenberg told Trump.
During the president's remarks, he suggested countries not only meet their minimum commitment of 2 percent but increase it to 4 percent, Sarah Huckabee Sanders, White House press secretary said in a statement. "So explain that", Trump said.
Foreign Affairs Minister Chrystia Freeland says Canada has significant concerns over the Nord Stream 2 pipeline that would bring gas from Russian Federation to the Baltic coast of Germany.
Berlin has given its blessing for the building of the Nord Stream 2 pipeline that will bring gas to Germany by way of the Baltic Sea. "We have a tremendous relationship with Germany".
"I am pleased to have this opportunity to be here for this exchange of views", she said, through a translator.
The two barely looked at each other during the few minutes journalists were allowed in the room.