KHARKIV, Ukraine — Russia’s new offensive across Ukraine’s northeastern border had been expected for months — yet it still surprised the Ukrainian soldiers stationed there to defend against it.

Ukraine’s 125th Territorial Defense Brigade — stretched thin along a roughly 27-mile stretch of the Kharkiv region’s border with Russia — used reconnaissance drones to monitor, daily, how Moscow was steadily building up forces for a possible attack. But the morning it happened, May 10, the brigade lost all its video feeds due to Russian electronic jamming.