The Second World War was the deadliest conflict in the history of humanity. Lasting from September 1, 1939 to September 2, 1945, it drew nearly all of the world's nations into two opposing coalitions and resulted in the deaths of between 60 and 75 million people, a figure that encompasses not only soldiers killed in battle but millions more who perished in massacres, genocides, famine, and disease. The Holocaust, the systematic murder of six million Jews and millions of others by Nazi Germany, stands as one of the defining atrocities of the modern age.
The war grew from failures left behind by the first. The Treaty of Versailles had imposed punishing terms on Germany, generating resentment that extremist movements exploited with devastating effectiveness. The rise of fascism in Europe and militarism in Japan created regimes that pursued territorial expansion through force. Japan invaded Manchuria in 1931 and launched a full-scale war against China in 1937. In Europe, Germany under Adolf Hitler annexed Austria and then the Sudetenland while Western powers, exhausted from the previous war and fearful of another, chose appeasement over confrontation.
The breaking point came on September 1, 1939, when Nazi Germany invaded Poland from the west. Britain and France, bound by treaty commitments, declared war on Germany. On September 17, the Soviet Union invaded Poland from the east under the terms of the secret Molotov–Ribbentrop Pact, and Poland was partitioned between the two powers. In 1940, Germany's military machine swept through Norway, Denmark, Belgium, Luxembourg, and the Netherlands in rapid succession. France, long considered the preeminent military power on the continent, fell in June 1940 after just six weeks of fighting. Britain stood alone, sustained by the Channel and by the determination of its government and people during the aerial Battle of Britain, the bombing Blitz, and the grinding naval struggle of the Atlantic.
The war expanded dramatically in 1941. In June, Germany launched Operation Barbarossa, a colossal invasion of the Soviet Union that opened the Eastern Front — the largest theater of the entire war, where the fighting would prove the most savage and the casualties the most staggering. In December, Japan attacked American and British territories across the Pacific and Asia, including a devastating strike on the US naval base at Pearl Harbor in Hawaii, which brought the United States into the war against the Axis powers of Germany, Italy, and Japan. Japan's early advances were breathtaking in their speed and scope, sweeping across much of coastal China and Southeast Asia, but its Pacific momentum was broken at the Battle of Midway in June 1942, when American naval and air forces destroyed four Japanese aircraft carriers in a single engagement.
The tide turned decisively in the winter of 1942 to 1943. At Stalingrad in the Soviet Union, German and Soviet forces fought one of history's most brutal urban battles; when it ended in February 1943, an entire German army had been encircled and destroyed. In North Africa, Axis forces were expelled after a prolonged campaign. An Allied invasion of Italy in July 1943 toppled the fascist regime of Benito Mussolini, though fierce fighting would continue on Italian soil until the war's end.
Allied power grew irresistibly through 1944. On June 6 — D-Day — American, British, Canadian, and other Allied forces landed on the beaches of Normandy in the largest amphibious operation in history, opening a front in Western Europe that would drive relentlessly toward Germany. The Soviet Union simultaneously launched massive offensives on the Eastern Front, sweeping westward through Central Europe. In the Pacific, American forces advanced island by island, crippling Japan's navy and seizing bases from which strategic bombing could reach the Japanese home islands. By early 1945, Germany was being squeezed from east and west simultaneously. Berlin fell to Soviet troops in late April, and Germany surrendered unconditionally on May 8, 1945.
Japan held out longer. On August 6, 1945, the United States dropped an atomic bomb on Hiroshima, annihilating much of the city. A second bomb fell on Nagasaki on August 9. The Soviet Union declared war on Japan and invaded Japanese-occupied Manchuria the same day. Faced with destruction on an unprecedented scale, Japan announced its surrender on August 15 and formally signed the surrender document on September 2, 1945, ending the war.
The world that emerged from the rubble was fundamentally different from the one that had entered the fighting. The United Nations was created to foster international cooperation and prevent future conflicts, with the victorious powers — China, France, the Soviet Union, the United Kingdom, and the United States — as permanent members of its Security Council. Europe lay shattered, its great powers diminished, and the process of decolonization accelerated across Asia and Africa. Two new superpowers, the United States and the Soviet Union, emerged from the war to face each other across an ideological divide, setting the stage for the Cold War that would define international politics for the next half century.
