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. Britain and France declared war two days later, but stood by as Poland fell in five weeks, partitioned between Germany and a Soviet Union that had signed the Molotov–Ribbentrop Pact a week before the invasion. 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 — the appeasement at Munich in 1938, where Chamberlain and Daladier handed Hitler the Czechoslovak Sudetenland in exchange for a promise, was the policy's epitaph. Hitler exploited every weakness — Anglo-French paralysis, Soviet opportunism, American isolation — racking up Denmark, Norway, the Low Countries and the fall of France in six weeks in the spring of 1940, then failing to break Britain in the air over the summer. He turned east while still at war in the west, invading the Soviet Union in June 1941 and committing Germany to the two-front war its general staff had dreaded since 1914. 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' strategic bombing, the U-boat war in the Atlantic, and the North African and Italian campaigns mattered and drained German strength, but they opened their decisive second front only at Normandy in June 1944, when the Wehrmacht was already broken in the east. Postwar Europe was the geography that this distribution of effort produced: Soviet armies stopped where they stopped, and the line they held — ratified at Yalta in February 1945 — hardened into the Iron Curtain, dividing the continent for forty-four years.
The European Union, NATO, the German Grundgesetz, the very concept of the post-war order, are all artifacts of the bargain struck at the end of this war: never again a great-power war on European soil, German power bound inside multilateral institutions, American security guaranteeing the whole. The current contest over the future of European security — Russia's war in Ukraine, an aging and openly questioned American security guarantee, a Germany rearming after eighty years of restraint — is essentially an audit of whether that bargain still holds, and of whether the continent can defend the settlement without the power that imposed it.