Second World War

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. Massacres, starvation, disease, and genocides—including the Holocaust—claimed millions of lives. When the Allies won, they occupied Germany, Austria, Japan, and Korea, and put German and Japanese leaders on trial for war crimes.

The war’s roots lay in unresolved tensions from World War I and the rise of fascism in Europe along with militarism in Japan. Key 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 mark the war’s beginning as September 1, 1939, when Nazi Germany, led by Adolf Hitler, invaded Poland. Britain and France then declared war on Germany. The Soviet Union also invaded Poland later that month, and the two powers split 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, the fighting continued mainly between Germany—now backed by Fascist Italy—and the British Empire and Commonwealth, with battles in the Balkans, the Mediterranean, the Middle East, East Africa, the aerial Battle of Britain, the Blitz, and the naval Battle of the Atlantic. By mid-1941, Axis forces 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 losses, including the crippling of its navy by the United States, the loss of key islands in the western Pacific, and defeats 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, followed by a Soviet invasion of 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 framework for international relations for the rest of the 20th century and beyond. The United Nations was created to encourage global cooperation and prevent future wars, with the victorious great powers—China, France, the Soviet Union, the United Kingdom, and the United States—becoming permanent members of its Security Council. The Soviet Union and the United States emerged as rival superpowers, laying the groundwork for the Cold War. Europe’s devastation weakened the influence of its major powers, triggering decolonization in Africa and Asia. Many countries with damaged industries turned toward economic recovery and expansion.

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, proposed start dates include the start of the Second Sino-Japanese War on July 7, 1937, or the earlier Japanese invasion of Manchuria on September 18, 1931. Other suggestions include the Italian invasion of Abyssinia on October 3, 1935. British historian Antony Beevor sees the Battles of Khalkhin Gol, fought between Japan and the forces of Mongolia and the Soviet Union from May to September 1939, as the true beginning. Some also view the Spanish Civil War as the start or a prelude to the conflict. 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 reunite. No formal peace treaty was ever signed between Japan and the Soviet Union, though the state of war ended with the Soviet–Japanese Joint Declaration of 1956, which also restored full diplomatic relations.

World War I had drastically changed Europe’s political map. The main Central Powers each lost territory in their peace treaties, and new nations formed from the collapse 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.