Hitler and Mussolini Declare War on the United States
Ultimately, it was Hitler and Mussolini’s declaration of war against the US on 11 December 1941, that overcame American opposition to a crusade against all the fascist powers.
Interestingly, Nazi Germany had signed no treaty that bound Hitler to support Japanese aggression. He was committed to taking on the US only in the event that America initiated hostilities.
What most drew Hitler into declaring war on the US was the very grandiosity of the thing. Not far beneath the surface, he was wildly excited that he was now the central figure in what had become the widest war in human history.
Hitler's declaration of war against the United States on 11 December 1941, four days after the Empire of Japan's attack on Pearl Harbor, Hawaii and six days after Nazi Germany's closest approach to Moscow, set him against a coalition that included the world's largest empire (the British Empire), the world's greatest industrial and financial power (the United States), and the world's largest army (the Soviet Union).
In late 1942, German forces were defeated in the second battle of El Alamein, thwarting Hitler's plans to seize the Suez Canal and the Middle East. In February 1943, the Battle of Stalingrad ended with the destruction of the German 6th Army. Thereafter came the Battle of Kursk. Hitler's military judgment became increasingly erratic, and Germany's military and economic position deteriorated. Hitler's health was deteriorating. His left hand trembled. Hitler's biographer Ian Kershaw and others believe that he may have suffered from Parkinson's disease. Syphilis has also been suspected as a cause of at least some of his symptoms, although the evidence is slight.
The U.S. Navy was already attacking German U-boats, and Hitler despised Roosevelt for his repeated verbal attacks against his Nazi ideology. He also believed that Japan was much stronger than it was, that once it had defeated the United States, it would turn and help Germany defeat Russia. So at 3:30 p.m. (Berlin time) on December 11, the German charge d'affaires in Washington handed American Secretary of State Cordell Hull a copy of the declaration of war.