World War II

World War II, also called the Second World War, lasted from September 1, 1939, to September 2, 1945. It was a worldwide struggle between two opposing alliances: the Allies and the Axis powers. Almost every nation on Earth took part, and many fought a total war of a kind never seen before. This conflict remains the deadliest in human history, with an estimated 60 to 75 million people killed, most of them civilians. Millions lost their lives through massacres, starvation, disease, and genocides, most notably the Holocaust. When the Allies won, they occupied Germany, Austria, Japan, and Korea, and put German and Japanese leaders on trial for war crimes.

The roots of the war lay in unresolved tensions from World War I and the rise of fascism in Europe alongside militarism in Japan. Important events leading up to it included Japan’s seizure of Manchuria in 1931, the Spanish Civil War, the start of the Second Sino-Japanese War in 1937, and Germany’s annexation of Austria and the Sudetenland. Most historians date the war’s beginning to September 1, 1939, when Nazi Germany, led by Adolf Hitler, invaded Poland. In response, the United Kingdom and France declared war on Germany. The Soviet Union also invaded Poland later that month, and the two powers divided the country under the Molotov–Ribbentrop Pact. In 1940, the Soviet Union took over the Baltic states and parts of Finland and Romania, while Germany conquered Norway, Denmark, Belgium, Luxembourg, and the Netherlands. After France fell in June 1940, fighting continued mainly between Germany—now joined by Fascist Italy—and the British Empire and Commonwealth. Battles erupted in the Balkans, the Mediterranean, the Middle East, East Africa, the skies over Britain (the Battle of Britain and the Blitz), and the naval Battle of the Atlantic. By mid-1941, the Axis had also defeated Yugoslavia and Greece. In June 1941, Germany invaded the Soviet Union, opening the Eastern Front.

In December 1941, Japan attacked American and British territories across Asia and the Pacific, including Pearl Harbor in Hawaii, which brought the United States into the war against the Axis. Japan overran much of coastal China and Southeast Asia, but its Pacific advance was stopped in June 1942 at the Battle of Midway. By early 1943, Axis forces had been beaten in North Africa and at Stalingrad in the Soviet Union. An Allied invasion of Italy in July led to the fall of its fascist government, and Allied offensives in the Pacific and the Soviet Union pushed the Axis back on every front. In 1944, the Western Allies invaded France at Normandy, opening another major front, while the Soviet Union moved into Central Europe. Japan also suffered heavy blows: its navy was crippled by the United States, it lost key islands in the western Pacific, and it was defeated in Burma. The war in Europe ended with the liberation of German-occupied lands and the invasion of Germany itself, culminating in the fall of Berlin and Germany’s unconditional surrender on May 8, 1945. On August 6 and 9, the United States dropped atomic bombs on Hiroshima and Nagasaki, and the Soviet Union invaded Japanese-occupied Manchuria. Japan announced its unconditional surrender on August 15 and signed a formal surrender document on September 2, 1945.

World War II reshaped the world’s political, economic, and social structures and set the foundation for international relations for the rest of the 20th century and beyond. The United Nations was created to encourage international cooperation and prevent future wars, with the victorious great powers—China, France, the Soviet Union, the United Kingdom, and the United States—holding permanent seats on its Security Council. The Soviet Union and the United States emerged as rival superpowers, leading into the Cold War. Europe’s devastation weakened its great powers, triggering decolonization in Africa and Asia. Many countries with damaged industries turned toward economic recovery and growth.

Historians generally agree that the war began with Germany’s invasion of Poland on September 1, 1939, and the British and French declarations of war two days later. For the Pacific War, suggested start dates include the beginning of the Second Sino-Japanese War on July 7, 1937, or the earlier Japanese invasion of Manchuria on September 18, 1931. Other proposals point to Italy’s invasion of Abyssinia on October 3, 1935, or the Battles of Khalkhin Gol between Japan, Mongolia, and the Soviet Union from May to September 1939. Some also view the Spanish Civil War as the start or a prelude to World War II. The war’s end date is also debated. At the time, many considered it over with the armistice of August 15, 1945 (V-J Day), rather than Japan’s formal surrender on September 2, 1945, which officially ended the war in Asia. A peace treaty between Japan and the Allies was signed in 1951. A 1990 treaty concerning Germany’s future allowed East and West Germany to reunify. No formal peace treaty was ever signed between Japan and the Soviet Union, though their state of war ended with the Soviet–Japanese Joint Declaration of 1956, which also restored full diplomatic relations.

World War I had radically changed Europe’s political map. The main Central Powers each lost territory in their peace treaties, and new nation-states formed from the breakup of the Austro-Hungarian, Ottoman, and Russian Empires. To prevent another world war, the League of Nations was created in 1920 by the Paris Peace Conference.