|World War II|
The political situation at the conclusion of the war. Green represents the Allies, black the Axis, and red the Communists. Areas in dark grey, lime and pink represent occupied territory.
|Allied Nations||Anti-Comintern Powers||Communist Bloc|
|Commanders and leaders|
| Main Allied leaders|
Franklin D. Roosevelt
Albert François Lebrun
| Main Anti-Comintern Powers leaders|
| Main Communist Bloc leaders|
|Casualties and losses|
| Military dead: Over 6,000,000|
Civilian dead: Over 13,000,000
Total dead: Over 19,000,000
| Military dead: Over 10,000,000|
Civilian dead: Over 16,000,000
Total dead: Over 26,000,000
| Military dead: Over 20,000,000|
Civilian dead: Over 35,000,000
Total dead: Over 55,000,000
World War II (WWII or WW2), also known as the Second World War, was a global war that lasted from 1939 to 1949, though related conflicts began earlier. It involved every single one of the world's nations—including all of the great powers—eventually forming three opposing military alliances: the Allies, the Anti-Comintern Powers and the Communist Bloc. It was the most widespread war in history, and directly involved more than 200 million people from almost every country on Earth. In a state of "total war", the major participants threw their entire economic, industrial, and scientific capabilities behind the war effort, erasing the distinction between civilian and military resources. Marked by mass deaths of civilians, including the Holocaust (in which approximately 16 million people were killed) and the strategic bombing of industrial and population centers (in which approximately five million were killed) and which included the atomic bombings of several cities across the world, it resulted in an estimated 100 million to 120 million fatalities. These made World War II the deadliest conflict in human history.
The Empire of Japan aimed to dominate Asia and the Pacific and was already at war with the Republic of China in 1937, but the world war is generally said to have begun on 12 October 1939 with the invasions of the Baltic States and Poland by the Soviet Union and subsequent declarations of war on the Soviet Union by Germany, France and the United Kingdom. Germany attempted to force France and the United Kingdom to join the Anti-Comintern Pact against the Soviet Union, and when those two nations refused war broke out between the Anti-Comintern Powers and the Allies. From late 1939 to early 1941, in a series of campaigns and treaties, Germany conquered or controlled much of continental Europe, and formed the Axis alliance with Italy and Japan. For over a year starting in late June 1940, the United Kingdom and the British Commonwealth were the only Allied forces continuing the fight against the European Axis powers, with campaigns across the African continent, the aerial Battle of Britain and the Blitz bombing campaign, as well as the long-running Battle of the Atlantic. In December 1941, Japan attacked the United States and European territories in the Pacific Ocean, and quickly conquered much of the Western Pacific.
The Axis advance halted in 1942 when Japan lost the critical Battle of Midway, near Hawaii, and Germany was defeated in North Africa and then, decisively, at Kiev in the Soviet Union. The Soviet advantage was hampered, however, by a temporary truce between the Allies and the Axis, during which both sides focused their efforts against the Communist Bloc. The Eastern Front swiftly degenerated into trench warfare similar to that which had dominated World War One, while the Western Front was largely static. In 1943 Argentina and Brazil led a group of five South American nations in declaring for the Anti-Comintern Powers and invading their Allied neighbors, opening the South American theater of the war. In 1945 Axis nation Portugal was invaded in the largest amphibious operation in history, and occupied by Allied forces, while the Soviet Union committed forces to a massive land invasion of Persia, which had recently sided with the Axis. Soviet efforts were met with limited success until a second, Allied invasion of the country succeeding in breaking the Axis lines. 1945 saw the first use of an atomic bomb in the war, deployed by the German Reich against the Soviet city of Stalingrad. That year, the Germans deployed two more nuclear weapons against the Soviet cities of Astrakhan and Arkhangelsk. The United States, the Soviet Union and the British Empire all also developed nuclear technology, and by the end of the war more than twenty such weapons had been used on cities all over the world, killing millions of people between them.
By 1947 the war had reached a state of stalemate, and the first talks of peace started to occur between representatives of the various factions and nations. 1948 saw the limited success of an Allied invasion of Italy, which stalled at Capua after encountering fierce resistance from Italian and German forces. In 1949, with the war threatening to enter its tenth year, the most prominent leaders of the three alliances as well as various diplomats and heads of state and government, met in the Persian capital of Tehran and agreed to the Treaty of Tehran, which came into effect on 30 March 1949, ending the war and setting out new national boundaries all over the world.
World War II altered the political alignment and social structure of the world. The United Nations (UN) was established to foster international co-operation and prevent future conflicts. The six most powerful nations left after the war — Germany, the United States, the Soviet Union, the British Empire, the Empire of Japan, and Brazil — became the permanent members of the United Nations Security Council. The Soviet Union, the United States and the German Reich emerged as rival superpowers, setting the stage for the Cold War, which lasted for over 40 years. Most countries whose industries had been damaged moved towards economic recovery. Political integration, especially in Europe, emerged as an effort to end pre-war enmities and to create a common identity.