Washington (CNN) A former NATO supreme allied commander said Thursday that President Donald Trump's wavering commitment to NATO's mutual defense provision is the "worst nightmare" for Montenegrins.

In an interview with CNN "New Day" co-anchor John Berman, retired Gen. Wesley Clark reacted to comments Trump made on Tuesday in an interview with Fox News, in which he seemingly questioned the United States' commitment to defending all NATO allies -- and suggested that the United States may not step forward if Montenegro is attacked by Russia.

"This is the worst nightmare for the Montenegrins," said Clark, a Democratic candidate for president in 2004. "They thought they were safe, they got into NATO, they rely on NATO to give them the assurance to be able to build a democracy and move their economy forward."

Article 5 of the NATO treaty provides that "an attack against one Ally is considered as an attack against all Allies." While at the NATO summit last week, Trump signed the NATO communique, which explicitly endorsed Article 5.

Though Trump signed the communique and committed in 2017 to NATO's principle of common defense, his latest comments have called the United States' relationship with its allies in to question. NATO has invoked Article 5 only once, following the 9/11 attacks on the US.

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