Can Zelensky Trust Trump? Ukraine’s Fate May Depend on the Answer.

For President Volodymyr Zelensky of Ukraine, a lot is riding on how much he can trust President Trump.

Mr. Trump offered only vague assurances on Monday that the United States would play a role in guaranteeing Ukraine’s safety if Mr. Zelensky were to cut a deal with President Vladimir V. Putin of Russia to stop the fighting.

“We’re going to make sure it works,” Mr. Trump said at the start of an hourslong meeting with Mr. Zelensky and a delegation of European leaders at the White House. “And I think if we can get to peace, it’s going to work. I have no doubt about it.”

The meeting ended with upbeat assessments by the Europeans and some progress toward a meeting between Mr. Zelensky and Mr. Putin, at a site to be determined. But the question of whether Mr. Trump can be trusted to keep his word speaks to his history of ever-changing positions and mercurial feelings about Ukraine and other diplomatic crises, especially when it comes to high-stakes negotiations.

It is a lot for a wartime leader like Mr. Zelensky to bet on, especially with his country three and a half years into a war instigated by Russia.

“They’re not just interested in any such assurances. They’re not interested in President Trump saying, ‘You can count on me,’” said William B. Taylor Jr., a former U.S. ambassador to Ukraine. “They’re not going to agree or accept an assurance, a political assurance. That doesn’t give them the security they need.”