With Luhansk Province firmly in Russia’s grasp after weeks of brutal fighting, the country’s military forces on Tuesday were already setting their sights on the next target. On the outskirts of Bakhmut, a key supply hub for Ukrainian forces, the Russian Army was ramping up its shelling as an apparent prelude for an inch-by-inch offensive into the province of Donetsk. In Sloviansk, one of the cities in Donetsk that lies in Russia’s path, the mayor, Vadym Lyakh, urged residents on Tuesday to flee the city, which he said was now on the front lines. “Artillery is already hitting the city,”...