President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE on Tuesday questioned why the U.S. must commit to defending all NATO allies during an interview following a contentious summit with the transatlantic alliance in Belgium.

Trump made the comment on Fox News's "Tucker Carlson Tonight" when host Tucker Carlson asked why should his "son go to Montenegro to defend it from attack?"

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"I've asked the same question," Trump said. "Montenegro is a tiny country with very strong people. ... They are very strong people. They are very aggressive people, they may get aggressive, and congratulations, you are in World War III.

"I understand, but that's the way it was set up. Don't forget, I just got here a little more than a year and a half ago, but I took over the conversation three or four days ago and I said you have to pay," Trump added, referencing the demands he made about defense spending during the summit.

Trump says he's bothered by provisions of NATO that require the US to come to the defense of other member countries. pic.twitter.com/2xY1FjBTSn — Aaron Rupar (@atrupar) July 17, 2018

Under NATO requirements, every nation must defend a fellow member nation if it is attacked.

CNN also notes that Trump signed the NATO communique as the meeting concluded last week, which explicitly endorses Article 5 — the article on defending NATO allies.

"Any attack against one Ally will be regarded as an attack against us all, as set out in Article 5 of the Washington Treaty," the communique reads. Article 5 has only been invoked after the 9/11 terrorist attacks in the U.S.

Trump said during a press conference before departing the summit in Belgium that NATO allies agreed to “substantially up” their defense spending commitments.