The decision left officials on both sides of the Atlantic scrambling to find out how long the pause might last.

Analysts say Ukraine’s European allies can fill some of the gaps and provide artillery systems. But they don’t possess alternatives to the U.S.-made HIMARS missiles and air defense systems, especially Patriots, which are crucial to help defend Ukrainian cities.

IN RECENT WEEKS Ukraine has endured some of the most intense drone and missile bombardments since the start of Russia’s full-on invasion three years ago. There could scarcely be a worse time for America to halt the delivery of precious air-defence equipment.