May 27 was a dark day for Ukraine. That was the day Lyman, the last free town north of the Donets River in eastern Ukraine’s Donbas River, finally fell to Russian forces. Capturing Lyman helped the Russian army to consolidate its position in Donbas and secure supply lines across the region. Lyman was a domino. As it fell, it knocked down Severodonetsk, the last free city east of the Donets. And as Severodonetsk fell, it toppled Lysychansk, its twin city on the opposite side of the river. Nearly four months later, the dominos are falling in the opposite direction. A...