Origins
The standard European origin is the German invasion of Poland on 1 September 1939, which led France and the United Kingdom to declare war on Germany two days later. The Asian theatre had begun earlier — the full-scale Japanese invasion of China began with the Marco Polo Bridge incident on 7 July 1937, with limited Japanese encroachment dating back to the Manchurian Incident of 1931.
The deeper causes included: the unresolved consequences of World War I, particularly the punitive terms of the Treaty of Versailles; the global Depression of the 1930s, which destabilized democratic governments and enabled the rise of fascist movements in Germany, Italy, and Japan; the failure of the League of Nations to enforce collective security; and specific aggressive policies by Nazi Germany under Adolf Hitler (1933–1945), Imperial Japan, and Fascist Italy under Benito Mussolini.
The European war
The early war (1939–1941) was a series of German victories: Poland (September 1939), Denmark and Norway (April 1940), the Netherlands, Belgium, Luxembourg, and France (May–June 1940). The Battle of Britain (July–October 1940) was Germany’s first significant operational failure. The German invasion of the Soviet Union on 22 June 1941 (Operation Barbarossa) opened the Eastern Front, which would become the largest land theater in human history.
The Eastern Front (1941–1945) involved roughly 80% of all German military deaths and the substantial majority of Soviet civilian deaths. Major engagements included the Siege of Leningrad (872 days, ~1 million civilian deaths), the Battle of Stalingrad (August 1942 – February 1943, ~2 million casualties), and the Battle of Kursk (July–August 1943, the largest tank engagement in history).
The Western Front reopened with the Allied landings at Normandy on 6 June 1944 (D-Day). Allied armies advanced through France, the Low Countries, and into Germany over the following 11 months. The German government surrendered unconditionally on 8 May 1945.
The Pacific war
The Japanese attack on the US Pacific Fleet at Pearl Harbor on 7 December 1941 brought the United States into the war the following day. The Pacific theatre was fought primarily by US, British Commonwealth, and Chinese forces against Japanese forces across the Pacific islands, Southeast Asia, and mainland Asia.
The decisive Allied advances included the naval battles of Midway (June 1942) and Leyte Gulf (October 1944), the island-by-island campaign through the Solomons, Marianas, and Philippines, and the strategic bombing of the Japanese home islands. The atomic bombs dropped on Hiroshima (6 August 1945) and Nagasaki (9 August 1945) killed approximately 200,000 people. The Soviet Union declared war on Japan on 8 August and invaded Japanese-held Manchuria. Japan surrendered on 2 September 1945, formally ending the war.
The Holocaust
The Nazi regime systematically murdered approximately 6 million Jews and several million additional victims (Roma, Soviet prisoners of war, Polish civilians, disabled people, political prisoners, gay men). The killing was conducted across an extensive network of concentration camps and extermination camps and through mass shootings by SS Einsatzgruppen on the Eastern Front. The Holocaust is the central genocide of the modern period and a defining moral fact of 20th-century history.
Consequences
The war reshaped the global order. The United States and the Soviet Union emerged as the two superpowers; the European colonial empires of Britain, France, the Netherlands, and Belgium collapsed within two decades. The United Nations was founded in 1945. The State of Israel was established in 1948. The Cold War (1947–1991) dominated the following four decades. The economic and technical transformations forced by the war — radar, nuclear weapons, computing, mass aviation, plastics, antibiotics — produced the modern world.