A big part of the problem in defending this part of Donbas, Ostap believes, is that the people who have stayed behind — the people who haven’t fled — don’t really believe they are part of Ukraine. In his view, the civilians who remain are all separatist sympathizers. He says they help the Russians navigate backcountry roads that aren’t on the maps. He asserts there have been instances of local collaborators getting caught providing information about Ukrainian troop movements or locations. Indeed, Slovyansk fell to Russian separatists in 2014: The retaking of the city by the Ukrainian military later that...