causes of World War II Second Italo-Ethiopian War Spanish Civil War Second Sino-Japanese War Soviet–Japanese border conflicts fascism in Europe aftermath of World War I Nazi Germany Adolf Hitler invaded Poland United Kingdom France declared war Molotov–Ribbentrop Pact Soviet Union Poland spheres of influence Finland Estonia, Latvia, Lithuania Romania campaigns treaties continental Europe Italy Japan North Africa East Africa fall of France British Empire Balkans Battle of Britain the Blitz Battle of the Atlantic an invasion of the Soviet Union Eastern Front theatre of war Axis Germany
The causes of World War II are debated, but contributing factors included the Second Italo-Ethiopian War, Spanish Civil War, Second Sino-Japanese War, Soviet–Japanese border conflicts, the rise of fascism in Europe, and European tensions in the aftermath of World War I. World War II is generally considered to have begun on 1 September 1939, when Nazi Germany, under Adolf Hitler, invaded Poland. The United Kingdom and France subsequently declared war on Germany on 3 September. Under the Molotov–Ribbentrop Pact of August 1939, Germany and the Soviet Union had partitioned Poland and marked out their "spheres of influence" across Finland, Estonia, Latvia, Lithuania and Romania. From late 1939 to early 1941, in a series of campaigns and treaties, Germany conquered or controlled much of continental Europe, in a military alliance with Italy, Japan and other countries called the Axis. Following the onset of campaigns in North Africa and East Africa, and the fall of France in mid-1940, the war continued primarily between the European Axis powers and the British Empire, with war in the Balkans, the aerial Battle of Britain, the Blitz of the United Kingdom, and the Battle of the Atlantic. On 22 June 1941, Germany led the European Axis powers in an invasion of the Soviet Union, opening the Eastern Front, the largest land theatre of war in history.
