Until it decided to confront Moscow with an existential military threat in Ukraine, Washington confined the use of American military power to conflicts that Americans could afford to lose, wars with weak opponents in the developing world from Saigon to Baghdad that did not present an existential threat to U.S. forces or American territory. This time—a proxy war with Russia—is different. Contrary to early Beltway hopes and expectations, Russia neither collapsed internally nor capitulated to the collective West’s demands for regime change in Moscow. Washington underestimated Russia’s societal cohesion, its latent military potential, and its relative immunity to Western economic...