Peering through an infrared scope, a Ukrainian soldier noticed some heads poking over a trench a few dozen yards away. “‘Are there any of our guys in front of us?’” he asked, according to an account of the ensuing firefight by fellow Ukrainian soldiers. There were not. Two Ukrainians crept forward into the muddy wasteland of artillery craters between the two trench lines outside the eastern city of Lyman, eventually reaching the wreckage of an armored personnel carrier. Using it as cover to shoot from an unexpected angle, they forced the Russians to retreat. When it was over, they found...