The start of the war is generally held to be September 1 1939, with the German invasion of Poland and subsequent declarations of war on Germany[4] by most of the countries in the British Empire and Commonwealth, and by France.[5][6] Many countries were already at war before this date, as a result of other events, and many who were not initially involved joined the war later. Like wikipedia says, there were many reasons- you should really have a read of the article.