On the battlefield, “Ukraine is not going to run out of bullets tomorrow,” said Michael A. Horowitz, a geopolitical and security analyst who is the head of intelligence at the Le Beck consultancy. “But a suspension of military aid would have a very noticeable impact on the conflict that would become visible in the coming months.”

The United States has given $65.9 billion in military aid to Ukraine since Russia launched its full-scale invasion in 2022. That’s far more than any other country (the second highest is Germany’s $13 billion, according to the Kiel Institute think tank.) In fact, the U.S. contributions are so large that they roughly equal all other nations’ aid put together.

A significant part of the American aid has been to supply Ukraine with air defense missiles, which not only protect Kyiv’s military positions but also civilians in cities, which have been targeted by the Russian military. It is also crucial in giving Ukraine artillery, which it has used to slow Russian advances over the winter. And it has been central in training the Ukrainians how to use this equipment, maintaining and repairing it, as well as providing transport.

Europe could pick up some of this slack, but it will be near impossible to do so completely, Horowitz said. And while removing American support would not cause issues immediately, in time “they will become quite crippling,” he added.

Likewise, though Ukraine receives some ammunition from Europe, “uncertainty about future U.S. aid shipments could lead Ukraine to begin rationing ammunition,” said Rob Lee, a senior fellow at the Foreign Policy Research Institute, a Philadelphia-based think tank.

Ultimately this “will lead to higher casualties for Ukrainian forces,” he told NBC News.

In the skies, Ukraine relies on the U.S. to supply the interceptor missiles for its Patriot air defense systems. “Without them, Ukraine will have more difficultly protecting its cities from Russian missile strikes,” Lee said.

It also has no effective replacement for the multiple guided missiles, which the U.S. supplies for its High Mobility Artillery Rocket System, or HIMARS, which have been used to great effect against Russia.

Added to that, “Ukraine already faces a manpower shortage, so the loss of U.S. aid will pose greater challenges for Ukraine,” Lee said.