Fascist movements rising atop the economic ruin, nationalist anger, and unresolved borders left by the First World War dragged Europe back into war in the 1930s. Hitler's invasion of Poland in September 1939 is taken as the conventional starting point. The six years that followed saw the conflict spread to every corner of the world: Europe, the Soviet Union, North Africa, the Pacific, China, Southeast Asia.

The Second World War carries two great historical weights. The first is the Holocaust: Nazi Germany's systematic extermination of six million Jews, along with more than ten million Roma, gay people, disabled people, and political opponents. Auschwitz-Birkenau, Treblinka, Sobibór, and the other camps stand as the only known case of an industrial-scale genocide, among the darkest facts in human history. The second is the atomic bombings of Hiroshima and Nagasaki in August 1945 — ending the war but opening the nuclear age and the existential risk that came with it.

The war's consequences ran deep. The United Nations and the Bretton Woods system were established. Europe's colonial empires began to unravel; India, Indonesia, Vietnam, Algeria, and dozens of countries gained independence over the next two decades. A bipolar world emerged: the Cold War between the United States and the Soviet Union became the spine of international relations for the next half-century. Germany, Japan, and Italy were democratised and rebuilt. Perhaps most importantly: the phrase 'a war to end all wars' had been said in 1918 — after 1945, at least a direct great-power war on a global scale did not occur again.