When was World War II?
World War II lasted from September 1, 1939 (Germany's invasion of Poland) to September 2, 1945 (Japan's formal surrender). In Asia, the conflict arguably began earlier with Japan's invasion of China in 1937. It was the deadliest conflict in history, killing an estimated 70–85 million people, including approximately 6 million Jews in the Holocaust.
World War II is conventionally dated from September 1, 1939, to September 2, 1945, though its Asian dimension began earlier and its aftermath extended long after the fighting stopped. These dates frame the deadliest and most geographically extensive conflict in human history.
The European war began when Germany invaded Poland on September 1, 1939, prompting Britain and France to declare war on September 3. But the conflict's Asian origins precede this. Japan had invaded Manchuria in 1931 and launched a full-scale war against China in July 1937, producing atrocities like the Nanjing Massacre. If the Asian theater is included, the war was already two years old when Germany attacked Poland.
The war's major phases unfolded on a staggering scale. Germany's blitzkrieg conquered Poland, Denmark, Norway, the Netherlands, Belgium, and France by mid-1940. Britain stood alone until Germany invaded the Soviet Union in June 1941 (Operation Barbarossa), opening the war's most destructive front. Japan's attack on Pearl Harbor on December 7, 1941, brought the United States into the conflict, globalizing the war. The year 1942 saw the war's turning points — the Soviet defense of Stalingrad, the American naval victory at Midway, and the British victory at El Alamein.
From 1943 onward, the Allies gradually gained the upper hand. Italy was invaded in 1943. D-Day (June 6, 1944) opened the Western Front in France. The Soviet Union drove westward through Eastern Europe. Germany surrendered unconditionally on May 8, 1945 (V-E Day). Japan fought on until the atomic bombings of Hiroshima (August 6) and Nagasaki (August 9), surrendering on August 15, with the formal ceremony on September 2, 1945, aboard the USS Missouri.
The scale of death was almost beyond comprehension. An estimated 70–85 million people died, making it the deadliest conflict in human history. The Soviet Union suffered the greatest losses — approximately 27 million dead. China lost an estimated 15–20 million. Poland lost roughly 17% of its entire population. The Holocaust murdered 6 million Jews and millions of others. The war ended with the dawn of the nuclear age, the beginning of the Cold War, and the creation of an international order — the United Nations, the Bretton Woods financial system, the decolonization movements — that continues to shape our world.