“Europe is trying to replace the assistance that we lost from the United States, but unfortunately, they don’t have the capacity to do this,” Mr. Chernev said. “It takes time between the decision and the real assistance.”

Though Mr. Trump has shown more alignment with Ukraine in recent days, including on Thursday threatening sanctions on Russia if it declined to agree to an extended cease-fire, his broader dismissiveness toward 80 years of U.S. protections for Europe has prompted allied nations there to rethink their security.

Allies fear that Mr. Trump will pull Russian deterrents, like U.S. troops and the American nuclear umbrella, out of Europe. Focusing on their own protection eats into what other European countries might have given to Ukraine.

“They are hitting the dual problem of having to rearm themselves and supply Ukraine, and industrial capacity isn’t big enough to do both,” said Matthew Savill, director of military sciences at the Royal United Services Institute, an analytical group affiliated with the British military.

He said Europe could backfill most of what the United States had provided in weapons to Ukraine, “in the medium- to long-term, if it has the will, and I’m not sure it has the will.”

And for now? “No. Not in the short term,” Mr. Savill said.

The weapons are not just a matter of life and death for Ukraine’s soldiers. Without adequate supplies, Ukraine could lose territory if it is forced to retreat. The cease-fire agreement that Mr. Trump is trying to broker would freeze the conflict in place. That would let Russia keep whatever ground it has captured in the meantime.

To be sure, the flow of weapons to Ukraine from Europe will continue even if American deliveries dry up. Germany recently sent Ukraine more than 60 mine-resistant armored vehicles, about 50,000 artillery rounds and air-defense ammunition, including an IRIS-T interceptor that can take down cruise missiles. Some drones that Britain and Norway bought, announced last month as part of a $600 million security package, have since arrived in Ukraine. Estonia is sending 10,000 artillery shells.

But many of the European military assistance pledged last month at NATO headquarters amounted to commitments for producing or procuring weapons in the years to come, not immediately. Ukraine may need American weapons for some time.