<P> 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 . 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 , and 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 . </P>
who started ww2 and how did it start
begun on 1 September 1939 with the invasion of Poland by Nazi Germany