Amid a faltering performance by President Biden in the presidential debate Thursday night, former President Donald J. Trump caused anxiety among America’s allies with a simple shrug.

Mr. Trump has regularly disparaged NATO and even threatened to withdraw the United States from it, and during the debate, he did nothing to assuage European concerns about his antipathy toward the military alliance.

Asked by Mr. Biden if he would pull out of NATO, Mr. Trump did not answer but shrugged.

“I was very worried prior to this debate and I’m even more worried now,” said Jana Puglierin, director of the German office of the European Council on Foreign Relations. “Trump may or may not want to leave NATO officially, but he has every means to undermine NATO.”

At the heart of NATO is Article Five of its charter, committing each member country to the defense of all the others. “Deterrence is all about credibility, and deep down, Article Five, has always been what you make of it,” Ms. Puglierin said. “So it depends on the U.S. president making it a credible threat.”

Given Mr. Trump’s skepticism about alliances, European nations that rely on the promise of American protection, she said, are worried he might try to forge bilateral relationships with Europe “and make them transactional.”

Camille Grand, a former assistant secretary general of NATO, said that in a second term, Mr. Trump would be surrounded by people “who want to turn his instincts into policy rather than saying, ‘This is a bad idea, Mr. President.’”