The two causes of ww2 was an increase in Germany militarism,caused by the rise of Adolf Hitler,and the political culture of alliances. There were however other causes such a weak league of nation. The immediate cause of ww2 was German invasion of Poland.
The USA did not join the war immediately it started. This was because of the isolation policy, as defined in the Monroe doctrine. Roosevelt,FD, had to wait until the public was as ready for the war as the nation. He came up with series of efforts to assist the allies before,japanese aggression led to the USA declaring war on japan. Consequently Germany declared war on the USA.
