Actually, Germany declared war on the US on December 11th, 1941 (one of the greatest head-scratchers in history; noone is entirely sure why - although you could say that the US was waging a de-facto war on (who else?) the U-boats from October 1941.)
The 7th was US on Japan (for obvious reasons)
First five U-boats arrived in American waters in mid-January 1942.