Germany declared war on the USA after the Japanese bombed Pearl. Churchill knew the Japanese was going to bomb Pearl but didn't warn the US because he knew that when the attack took place it would bring the US into the fray and that Hitler was stupid enough to declare war on the US after the sneak attack on Pearl. Thus England would be guaranteed the US as an ally, for which he had been begging Roosevelt for months on end. The US had been isolationist since after WW1, seeing no advantages to being embroiled in yet another European conflict. They knew the Atlantic Ocean kept them safe from the Hun so were not worried about it. Then the Nipponese pulled their sneak attack on Pearl and Bob's your Uncle.