World War II was a global war that lasted from 1939 to 1945, although related conflicts began earlier. It involved the vast majority of the world’s nations—including the great powers— that eventually formed opposing military alliances: the Allies and the Axis. It was the most widespread war in history and directly involved more than 100 million people from over 30 countries. In a state of “total war,” the major participants threw 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 11 million people were killed) and the strategic bombing of industrial and population centers (in which approximately one million were killed, including the atomic bombings of Hiroshima and Nagasaki), the war resulted in an estimated 50 to 85 million fatalities. These made World War II the deadliest conflict in human history.
The world war is generally said to have begun on September 1, 1939 with the invasion of Poland by Germany and subsequent declarations of war on Germany by France and the United Kingdom. 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. Germany and the Soviet Union partitioned and annexed territories of their European neighbors: Poland, Finland, Romania and the Baltic states. The war continued primarily between the European Axis powers and the coalition of the United Kingdom and the British Commonwealth, with campaigns in Africa, the aerial Battle of Britain, the Blitz bombing campaign and the Balkan campaign, as well as the long-running Battle of the Atlantic. In June 1941, the European Axis powers launched an invasion of the Soviet Union, opening the largest land theater of war in history, that trapped the major part of the Axis’ military forces into a war of attrition. In December 1941, Japan, already at war with China, 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 Stalingrad in the Soviet Union. In 1943, with a series of German defeats on the Eastern Front, the Allied invasion of Sicily and Italy, and Allied victories in the Pacific, the Axis lost the initiative and undertook strategic retreat on all fronts. In 1944, the Western Allies invaded German-occupied France, while the Soviet Union regained all of its territorial losses and invaded Germany and its allies. During 1944 and 1945 the Japanese suffered major reverses in mainland Asia in south central China and Burma, while the Allies crippled the Japanese Navy and captured key Western Pacific islands.
The war in Europe concluded with an invasion of Germany by the Western Allies and the Soviet Union, culminating in the capture of Berlin by Soviet and Polish troops and the subsequent German unconditional surrender on May 8, 1945. Following the refusal of Japan to surrender, the United States dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki on August 6th and 9th respectively. With an invasion of the Japanese archipelago imminent, the possibility of additional atomic bombings and the Soviet Union’s declaration of war on Japan and invasion of Manchuria, Japan surrendered on August 15, 1945. Thus ended the war in Asia, cementing the total victory of the Allies.
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 victorious great powers—the United States, the Soviet Union, China, the United Kingdom, and France—became permanent members of the UN Security Council. The Soviet Union and the United States emerged as rival superpowers, setting the stage for the Cold War that lasted for the next 46 years. Meanwhile, the influence of European great powers waned, while the decolonization of Asia and Africa began. Political integration, especially in Europe, emerged as an effort to end pre-war enmities and to create a common identity. (from me and Wikipedia)