The traditionally Russian-speaking area is at the heart of what the Russian president calls the “root causes” of the war, and taking it over is near the top of his list of territorial and political demands.The proposal to end the war in Ukraine that emerged from the summit in Alaska between President Trump and President Vladimir V. Putin of Russia centers on persuading Kyiv to give up the Donbas, the industrial region in the east.The traditionally Russian-speaking area has been at the heart of what Mr. Putin calls the “root causes” of the war, and taking it over is near...