On Oct. 19, Ukrainian marines motored across the Dnipro River and, in a series of infantry actions in and around the settlement of Krynky, secured a bridgehead on the otherwise Russian-occupied left bank of the river. Winning, and holding, a bridgehead is a first step for Ukraine’s southern command as it tries to sustain the country’s southern counteroffensive—which kicked off in June but has idled in recent weeks—and set conditions for a possible push deeper into Russian-held southern Kherson Oblast. The gateway to occupied Crimea. The Kremlin knows it. And we now have seen the first thin evidence in several...