Claim: World War II was fought between two military alliances.

World War II -LRB- often abbreviated to WWII or WW2 -RRB- , also known as the Second World War , was a global war that lasted from 1939 to 1945 , although related conflicts began earlier . It involved the vast majority of the world 's countries -- including all of the great powers -- eventually forming two 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 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 -LRB- in which approximately 11 million people were killed -RRB- and the strategic bombing of industrial and population centres -LRB- in which approximately one million were killed , and which included the atomic bombings of Hiroshima and Nagasaki -RRB- , it resulted in an estimated 50 million to 85 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 1 September 1939 with the invasion of Poland by Nazi Germany and subsequent declarations of war on Germany by France and the United Kingdom . Supplied by the Soviet Union , 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 . Under the Molotov -- Ribbentrop Pact of August 1939 , Germany and the Soviet Union partitioned and annexed territories of their European neighbours , 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 including the North Africa and East Africa campaigns , the aerial Battle of Britain , the Blitz bombing campaign , the Balkan Campaign as well as the long-running Battle of the Atlantic . On 22 June 1941 , the European Axis powers launched an invasion of the Soviet Union , opening the largest land theatre of war in history , which trapped the major part of the Axis ' military forces into a war of attrition . In December 1941 , Japan attacked the United States and European colonies 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 the Allied invasion of Italy which brought about Italian surrender , 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 troops and the subsequent German unconditional surrender on 8 May 1945 . Following the Potsdam Declaration by the Allies on 26 July 1945 and the refusal of Japan to surrender under its terms , the United States dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki on 6 August and 9 August 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 15 August 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 -LRB- UN -RRB- 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 the permanent members of the United Nations Security Council . The Soviet Union and the United States emerged as rival superpowers , setting the stage for the Cold War , which lasted for the next 46 years . Meanwhile , the influence of European great powers waned , while the decolonisation of Asia and Africa began . 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 .