A hail of shrapnel from kamikaze drones ripped through the tent where off-duty Ukrainian border guards were sleeping near a crossing with Belarus, three hours north of Ukraine’s capital. Viktor Derevyanko woke to scalding pain, his body burning. Blood spilled from his hand as he tried to wipe his face. A piece of metal had traveled through his arm and stomach and into the muscle around his heart. “I couldn’t get my bearings,” said Derevyanko, the deputy head of the unit. “Only on the third explosion did I manage to fall out of bed and try to find at least...