Ukraine’s most heavily fortified region has become the center-point for peace talks to end the war with Russian leader Vladimir Putin reportedly demanding that Kyiv cede the entire territory to Moscow. But the Donestk oblast — which is smaller than Massachusetts and home to about 4 million people — has been at the center of some of the most brutal fighting of the war, and the Kremlin has not been able to take the entire region after more than three and a half years. A source familiar with Friday’s meeting between Putin and President Trump described negotiations over the fate...