Foreseeing the possibility of an American retreat from the world if Mr. Trump returns to office, Congress recently passed legislation barring any president from withdrawing from the NATO treaty without Senate approval. But Mr. Trump would not even need to formally quit the alliance to render it pointless.

And if the United States could not be counted on to come to the aid of partners in Europe, where it has the strongest historic ties, then other countries with mutual security agreements with Washington like Japan, the Philippines, Thailand, Australia, Argentina, Brazil, Chile, Colombia, Costa Rica and Panama could hardly be sure of American help, either.

Peter D. Feaver, a Duke University professor and former national security aide to Mr. Bush and President Bill Clinton, said Mr. Trump could reduce American troops in Europe to a level that “would render any military defense plans hollow” and “regularly poor-mouth the U.S. commitment” in a way that would convince Mr. Putin that he had free rein.

“Just doing those two things could wound and perhaps kill NATO,” Mr. Feaver said. “And few allies or partners in other parts of the world would trust any U.S. commitment after seeing us break NATO.”