By Sophie Tatum, CNN

(CNN) -- President Donald Trump seemingly questioned the United States' commitment to defending all NATO allies in an interview that aired Tuesday evening.

At the end of a major international trip with three stops -- a meeting with Russian President Vladimir Putin in Helsinki, Finland, a visit to the United Kingdom and the latest NATO summit in Brussels, Belgium -- Trump spoke with Fox News' Tucker Carlson, where he appeared to waver on whether the US would come to the defense of all NATO member countries.

NATO requires all members to help defend fellow member nations that have been attacked, which Carlson noted to Trump.

"Why should my son go to Montenegro to defend it from attack?" Carlson inquired.

Trump responded: "I've asked the same question. 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," he continued. "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."

While at the NATO summit last week, Trump signed the NATO communique, which explicitly endorsed Article 5.

"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.

In 2017, Trump said he agreed with the commitment that members will come to one another's aid if they're attacked, which is Article 5 of the NATO charter. NATO has invoked Article 5 only once, following the 9/11 attacks on the US.

"I am committing the United States to Article 5," Trump said at the time.

"And certainly we are there to protect," he then added, saying this is why the US is "paying the kind of money necessary to have that force."

"Yes, absolutely I would be committed to Article 5," he said at the time.

CORRECTION: This story has been updated to reflect that NATO invoked Article 5 following the 9/11 attacks.