Today in 1941, Adolf Hitler declared war on America. It was four days after the attack on Pearl Harbor. The United States, formally a neutral party in the War, was now officially a participant.
Germany was not in on the plot to bomb Pearl Harbor. Germany was, however, bound to back Japan if it were to go to war with the United States. After some negotiation, at least. Japanese Ambassador Oshima visited the German Foreign Minister von Ribbentrop on December 8 to pressure the Nazis to declare war against the US. The Germans were not immediately onboard. They had an out - the language of the Tripartite Pact, which pledged German support of Japan if they were attacked, afforded an out for the Nazis, as Japan launched the attack. Von Ribbentrop was apprehensive, correctly, that America's entrance into the hostilities could tip the balance against Germany.
Hitler believed that an American declaration of war against Germany was imminent. This was not an irrational intuition. The Americans had been attacking U-boats. FDR was also rhetorically aggressive against the Nazis.
Hitler's decision to declare war was also informed by an inaccurate assessment of Japan's military strength. He believed that Japan would have adequate might to defeat America and then join Germany in the fight to conquer Russia. The declaration was issued, and later in the day, Hitler spoke before the Reichstag to announce the move and justify it.
In his speech, Hitler laid the blame at FDR's feet, claiming that his New Deal programs were a poorly disguised failure, backed by Jews. "First he incites war, then falsifies the causes, then odiously wraps himself in a cloak of Christian hypocrisy and slowly but surely leads mankind to war."