After weeks of fighting for scraps of territory on the war’s bloodiest front, Oleh, a 21-year-old Ukrainian company commander, was summoned suddenly last August, along with thousands of other soldiers, to an obscure rendezvous point in the Kharkiv region. At his last position, relentless Russian artillery fire had stalked his men’s every step. But here, in a patch of villages, farmland and streams in Ukraine’s northeast, the quiet was deeply alarming. “The silence bothered me the most,” Oleh said. “It seemed off. How could this be?” Even more unsettling were the orders his superiors handed down: to charge as far...