Mr. Trump is not the only USA president to urge allies to increase military spending, but he is the first to not explicitly commit to NATO's collective defense clause: "An attack on one is an attack on all". This comes two days after the president told members for not spending enough on defence.
"My feeling is that we agreed on many areas". During last year's Republican primary campaign, Trump alleged, "NATO is costing us a fortune" and is "obsolete" because other nations aren't pulling their weight.
President Trump speaking next to NATO Secretary General Jens Stoltenberg during the opening of the new NATO headquarters. Declining to do so is significant as Trump has threatened to remove the United States from the alliance, which was formed in 1949.
Trump announced a review of "deeply troubling" USA intelligence leaks over the Manchester bombing, in which 22 people died, and warned that those responsible could face prosecution, the White House said. "I want to tell you that it is natural for the president of the United States to be in the first row".
It's a reminder of NATO's commitment to its collective defense clause - so called Article 5 - which commits allies to defend any of the 28 members that come under attack.
But one senior diplomat said Trump, who left the leaders' dinner before it ended to fly to Italy for Friday's Group of Seven summit, said the remarks did not go down well at all.
The meeting comes after the president told North Atlantic Treaty Organisation members Thursday in Brussels that they must "finally contribute their fair share", saying 23 North Atlantic Treaty Organisation countries are not paying what they should be paying on defense.
Mr Markovic himself, however, shrugged off the slight.
Standing before a large piece of wreckage from the World Trade Center that will serve as a memorial at the headquarters, he vowed to "never forsake the friends that stood by our side" after the Sept 11, 2001, attacks, a pledge White House officials called an affirmation of mutual defence.
"We shouldn't be subsidizing other nations", he said on Twitter.
European Council President Donald Tusk admitted on Thursday that the bloc is still at odds with the United States over issues like climate, trade and Russian Federation.
Tusk said he is not "100 percent sure" that he and Trump "have a common opinion about Russia" but said they share the same beliefs about the conflict in Ukraine.
After meeting Trump on Thursday in Brussels, Tusk told a news conference: "It was clear for both of us that the European Union at 27 is more united after Brexit than before Brexit".