US Warships were involved prior to Dec 7 1941. The USS Niblack attacked a German U Boat on 10 April 1941. The USS Niblack was involved in the US occupation of Iceland. US warships provided escort to convoys to England prior to Dec 1941.
Before the actual attack by the Japanese on Pearl Harbour, of which the Americans state they had no foreknowledge, the US Navy attacked and sank a midget submarine trying to enter Pearl Harbour. Since the Japanese had not actually attacked as yet the Americans actually attacked first without declaring war. The AVG, US pilots and P-40 aircraft, was set up 15th April 1941 by Roosevelt to provide support to China against the Japanese.
June 1941 the US (3000 odd men) occupied Iceland. Iceland had been invaded by Britain in May 1940. The Iceland government protested the British Invasion. Britain handed over the control of Iceland to the USA in June 1941.
Americans fought for the British during the battle of Britain.
Of common knowledge is Lend Lease.
Read more:
http://wiki.answers.com/Q/When_did_the_US_enter_World_War_2#ixzz1Aq5AIv00
