After Nazi Germany’s Tripartite Pact partner, Japan, bombed Pearl Harbor on Dec 7th 1941, Adolph Hitler seized the opportunity to declare war on the US on December 11th, 1941. He knew that he was going to eventually fight the US at some point, and Japan had the world class surface Navy that he did not.
So at that point, a major world power like Nazi Germany, who possessed a significant submarine threat to US merchant shipping posed an existential threat to this nation’s continued economic existence. Declaring war on Nazi Germany was the only rational option.
Was going to happen anyway, since there were no longer restrictions on aiding Britain, because they had also declared war on Japan.
Pretty much after the sinking of The Reuben James a few months earlier, any illusions that we were going to avoid war with Germany were pretty much shattered.