Over the last five days of May, Ruslan, an English teacher in a Russian town near the Ukrainian border, heard the distinct sound of a multiple rocket launcher strike for the first time. Shelling would begin around 3 a.m., sometimes shaking his house, and continue through the morning. He had heard the thud of explosions in distant villages in the past, he said, and in October shelling damaged a nearby shopping mall. But nothing like this. “Everything changed,” he said. Fifteen months after Russian missiles first roared toward Kyiv, residents of the Russian border region of Belgorod are starting to...