The Russian president, Vladimir V. Putin, has repeatedly said that the fall of the Donbas region in eastern Ukraine is inevitable and will happen faster than Kyiv thinks. Recent trends on the battlefield paint a different picture. A monthslong Ukrainian counteroffensive in the northern part of the Donbas has thwarted Russian efforts to encircle the remaining Ukrainian-controlled cities in the region. Russia, Western analysts say, has lost dozens of square miles of territory during the Ukrainian campaign, which Kyiv is calling Operation Vivaldi. In the southern Donbas, Russia continues to mount furious assaults to break through Ukrainian lines. But Kyiv...