On September 1, 1939, German armoured columns crossed into Poland and forced the world to find out, again, what total war between industrialized states actually looked like. By the time it ended in May 1945, a third of European Jewry was dead, the Soviet Union had lost twenty-seven million people, the European empires were financially broken, the United States was the only intact great power left, and the geographical question of where Europe began and ended had been re-set on terms it would live with for half a century.
The war in Europe was, in a sense, a resolution of the unfinished business of the Great War: the Versailles system never settled the German question, and the inter-war democracies never developed the will to enforce it. Hitler exploited every weakness — Anglo-French paralysis, Soviet opportunism, American isolation — until he attacked the wrong country at the wrong time. The decisive theatre, contrary to the way the war is often remembered in the West, was the Eastern Front: roughly 80 percent of German army casualties were inflicted by the Red Army, in a four-year war of unspeakable savagery that the Russians still call the Great Patriotic War. The Western Allies opened their second front in June 1944, when the Wehrmacht was already broken. Postwar Europe was the geography that fact produced: Soviet armies stopped where they stopped, and the line they held became the Iron Curtain.
The European Union, NATO, the German constitution, the very concept of the post-war order, are all artifacts of the bargain struck at the end of this war. The current contest over the future of European security — Russia, Ukraine, an aging American security guarantee — is essentially an audit of whether that bargain still holds.