Critics of Mr. Biden said he has no one to blame for his diplomatic challenges in Europe but himself for an inconsistent approach to international crises.

“The contradiction, I think, is in American policy,” said Peter Rough, the director of the Center on Europe and Eurasia at the Hudson Institute and a former aide to President George W. Bush. “In Ukraine, he’s backing Ukraine against the Russian-Iranian alliance, while in Gaza he is managing Israel, even limiting it, as it confronts an Iranian proxy.”

From the other side of the spectrum, some foreign policy veterans said Mr. Biden has brought troubles upon himself by being too supportive of Israel.

“I am not at all sure that Biden has made the right choices on Israel-Gaza, although I acknowledge that he’s in a tough spot, as is our country,” said Eric Rubin, a longtime U.S. diplomat and the former president of the American Foreign Service Association. “Israel has lost the sympathy of most other countries and their citizens, and we won’t see it get it back in our lifetimes, I fear.”

But at the end of the day, some diplomats said, France and the other allies ultimately defer to the United States when it comes to such issues. And even though he will find them on different pages, Mr. Biden enjoys a constructive relationship with his peers, unlike his predecessor, and possible successor, Donald J. Trump, who berated European allies over their disagreements and left them dreading his potential return to office.

“The United States still plays the indispensable role,” said Mr. Daalder. “Everybody is looking to us to figure out how do we deal with Russia, how do we deal with China, and frankly even how do we deal with Israel. We’re still looked at by our friends and by our adversaries as the ones who will determine the outcome.”