Ukraine’s success illuminates a strategy that has allowed a smaller state to—so far—outlast a larger and much more powerful one. Battles reveal more than they decide. Battles in which the outcome is truly up for grabs are rare, and battles that prove decisive in achieving a political goal are rarer still. Instead, battles demonstrate how effectively combatants planned, prepared, and executed before the fighting began. The result of a battle exposes not only how well matched the sides are but also how the war might unfold in the future. In that sense, the outcome of the Battle of Kyiv was...