DEBALTSEVE, Ukraine – A sweeping vista of snow-capped hills and an icy lake, the entrance to Debaltseve is always striking in winter. But this winter the hills are torn up by the track marks of armored vehicles, and the bridge across the lake punctuated by pillboxes.

Debaltseve is a city under siege, the bulge in Ukraine’s shaky front line against separatist militants equipped, trained and often sent from Russia.