World War II (1939): Poland invaded

Japan's war in China and Italy's wars in Africa preceded the European general war. Germany's attack on the Soviet Union in June 1941 and Japan's strike on Pearl Harbor on 7 December 1941 fused those wars into a single global contest. Lend-Lease, the Atlantic Charter and later the United Nations Declaration (1 January 1942) turned a set of bilateral fights into a coalition with shared -- if unequal -- aims.

Codebreaking at Bletchley Park and Station HYPO, radar, and the B-29 and V-2 programmes show how laboratories entered the order of battle without turning this page into a weapons manual. Nazi rule in Europe organised plunder, slave labour and the Holocaust: about six million Jews murdered, alongside millions of other targeted civilians. Japanese occupation in China and Southeast Asia produced mass death through massacre, famine and forced labour.

Resistance, collaboration and civil war inside occupied countries mean "the home front" was often a second front. Yalta and Potsdam framed occupation zones, while the Tokyo and Nuremberg tribunals tried a new language of crimes against peace and humanity. The 1945 UN Charter promised collective security that the Cold War immediately strained.