The Russian air force had deployed three times as many fighters and attack planes as the Ukrainian air force had in its entire inventory. Ukraine’s air defenses were disorganized and, in the case of certain key long-range radars, sitting out in the open where the Russians easily could target them. The Russians had a firepower advantage. The Ukrainians had the same advantages every defender possesses over an invader: motivation, simpler logistics, familiar terrain. Either side might’ve prevailed—the Russians by dominating the air, the Ukrainians by preventing the Russians from dominating the air. We know how it turned out. The Russian...