It was apparent months ago that the Kremlin’s intensive focus on Donbas risked creating vulnerabilities elsewhere. At the peak of the fighting in Donbas in early July, three-quarters of the Russian army’s roughly 110 front-line battalions in Ukraine were in Donbas. Just a handful of battalions defended Russian gains in and around Kherson. The Ukrainian army also concentrated its best forces in Donbas, of course—but not at the expense of the southern front. In late May, Ukrainian troops began pushing south toward Kherson, then 40 or so miles from the line of contact. Two months later, Kyiv’s southern counteroffensive has...