Washington (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."

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