Four-year-old Elizaveta Dmytrieva grinned as she pushed her stroller along the street in Vinnytsia, Ukraine. It was nearly five months into the war, but the city where her family had fled from Kyiv seemed safe enough. Her mother took an Instagram video as “Liza,” who was born with Down syndrome, led the way in a moment of delighted independence. Barely an hour later, the little girl was dead, her mother severely injured. And the image of her black and pink stroller, flipped on its side and spattered with blood, would become symbolic of the gruesome toll Russia’s invasion has inflicted...