They attacked US-flagged ships in the Atlantic in early 1942. This is an act of war.
But before that, Germany was operating a network of spies inside our borders. Go research “Duquesne Spy Ring.”
Italy and Germany both formally declared war on the U.S. on Dec. 11, 1941. Their rationale was that, as members of the Tripartite Pact with Japan, they were obligated to come to the aid of a member who was attacked. IOW, they were about as logical as Ahmadinejad, Mousavi and the Ayatollah.