Why was the Second World War a world war? One obvious explanation is that, by the end, it stretched across Eurasia, bringing together theatres of conflict as far apart as northern Africa, eastern Europe, the Pacific islands and China. No inhabited continent was entirely untouched, even if some escaped the direct effects of warfare. Yet at one moment in late 1941, it still looked possible that the conflict might not become truly global. Between the summers of 1939 and 1941, the western European powers were engaged in a vicious war confined almost entirely to their own continent. Britain and France had empires, but they were mostly sources for troops and resources, not front lines in themselves. Public opinion in the United States was doing its best to keep the country out of the Anglo-French war against the Nazis. The USSR went further and signed a non-aggression pact with Hitler that looked suspiciously like an alliance. Meanwhile, war had been raging between China and Japan since July 1937, but apart from being given some discreet military and financial assistance by the Americans, Soviets and British, the Chinese were essentially left to fight on their own. The European and Asian wars were on opposite sides of the Eurasian continent and there was no compelling reason why they would meet any time soon.

In June 1941, the Nazis reversed course with shocking speed, launching Operation Barbarossa against the USSR. Yet even following this, the war in Europe did not become global: Japan had signed a neutrality pact with the Soviets, allowing Stalin to think about his European flank without having to