President Trump’s freeze on U.S. aid going to Ukraine threatens to create severe problems for Kyiv in its war against Russia, leaving the country more vulnerable to aerial attacks on population centers and key infrastructure.

Experts predict the pause could be felt within a matter of weeks, depending on its scope.

“In terms of the immediate battlefield effect, this is bad,” said George Barros, an expert on the Ukraine-Russia conflict for the Institute for the Study of War.

Barros said the freeze will have “a significant effect” that will become more apparent should the suspension stretch into summer, when ammunition supplies are likely to dwindle. But the biggest gap will be felt when the Ukrainians run low on U.S.-made Patriot interceptors and other air defenses, which could happen within weeks.

“There are certain niche systems and capabilities that the United States provides Ukraine that the Ukrainians do not have an analogue for, and neither do the Europeans, particularly air defense interceptors,” he said.

Retired Rear Adm. Mark Montgomery, a senior fellow with the Washington-based think tank Foundation for Defense of Democracies, said there is simply no European substitute for U.S. air defense munitions such as Patriot interceptors and Advanced Medium-Range Air-to-Air Missiles — used to defend from incoming Kremlin drone and missile attacks on cities. Nor is there an easy replacement for long-range strike weapons such as HIMARS and ATACMS, which Ukrainian troops have used to strike targets just over the border in Russia.

“Their critical infrastructure is getting hammered by cruise and ballistic missiles,” Montgomery said. “Without these weapons systems, more Russian stuff hits, more infrastructure is damaged. Without the long-range strike, Russia can bring contact to the enemy quicker with less risk. This [freeze] makes things worse.”

Jennifer Kavanagh, senior fellow and director of........

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