The long-awaited counteroffensive last year failed. Russia has recaptured Avdiivka, its biggest war gain in nine months. President Volodymyr Zelensky has been forced to quietly acknowledge the new military reality. The Biden Administration’s strategy is now to sustain Ukrainian defense until after the U.S. presidential elections, in the hope of wearing down Russian forces in a long war of attrition. This strategy seems sensible enough, but contains one crucially important implication and one potentially disastrous flaw, which are not yet being seriously addressed in public debates in the West or Ukraine. The implication of Ukraine standing indefinitely on the defensive—even...