World War II (1 September 1939 – 2 September 1945) was a global conflict between the Allies and the Axis powers, involving most nations and resulting in the deadliest war in history, with 70 to 85 million deaths, over half of which were civilians. It featured total war, with major roles played by tanks, aircraft, and the first use of nuclear weapons. Civilians suffered from genocides like the Holocaust, as well as massacres, starvation, and disease. After the Allied victory, leaders of Germany and Japan were tried for war crimes, and their countries were occupied.

The war’s causes included unresolved issues after World War I, the rise of fascism in Europe, and militarism in Japan. Key precursors were Japan’s invasion of Manchuria (1931), the Second Sino-Japanese War (1937), and Germany’s annexations of Austria and the Sudetenland. The war began on 1 September 1939, when Nazi Germany, led by Adolf Hitler, invaded Poland, prompting Britain and France to declare war. Poland was pided under the Molotov–Ribbentrop Pact between Germany and the Soviet Union. By 1940, Germany controlled much of Europe, fought Britain in the Battle of Britain, and formed the Axis alliance with Italy and Japan.

In 1941, Germany invaded the Soviet Union, opening the Eastern Front. That same year, Japan attacked Pearl Harbor, drawing the United States into the war. Japan conquered much of Asia but faced setbacks in 1942, including defeat at the Battle of Midway. The Allies gained momentum in 1942–1943, defeating Axis forces in North Africa, at Stalingrad, and through invasions of Italy. In 1944, the Allies invaded Normandy in western Europe, while the Soviets advanced in the east. The war in Europe ended with the fall of Berlin and Germany’s surrender on 8 May 1945.

In the Pacific, Japan’s resistance ended after the U.S. dropped atomic bombs on Hiroshima and Nagasaki in August 1945, combined with the Soviet invasion of Manchuria. Japan surrendered on 2 September 1945, concluding the war.

World War II reshaped global politics, giving rise to the United Nations and rival superpowers, the United States and the Soviet Union, leading to the Cold War. European devastation accelerated the decolonization of Africa and Asia, while global economies began recovery and expansion. The war’s impact defined much of the 20th century and beyond.