Shortly after the attack on Pearl Harbor, the United States declared war on Japan. Being that Japan was a member of the Axis powers (Germany, Italy, and Japan) during the second world war, it was only a matter of time before Germany declared war on the United States. December 11th, 1941 Germany declares war on the United States.
Let me set the stage before we delve too deep. The United States during both world wars started out as neutral. At the time, both conflicts seemed to be a matter for only European powers and the United States was firm in its isolation before World War 1 and joined in that conflict only nineteen months before the end. After the war, the United States removed itself from the affairs of Europe once again by repudiating the Treaty of Versailles. Coupled with the Great Depression, the United States went back to its policy of isolationism. We had our own problems to worry about. We could not be bothered with European affairs.
What we know now about Nazi Germany, was not well known at the time. News of the death camps and mass genocide was not common knowledge during the 1940's. It's easy to take for granted now, with the internet and information so widely available, but in the 1940's secrets were much easier to keep than today. The United States was unconcerned with the affairs of Europe once again. While we supported our European allies, we were considered a neutral power. All that changed on December 7th, 1941. With the attack on Pearl Harbor and the subsequent declaration of war on Japan, Germany who was actually surprised by Japan's actions towards the United States had no choice but to declare war in support of their Japanese allies.
To this day, the events of the second world war affect us greatly. How long would it have taken us to get the nuclear weapon if the war hadn't forced its necessity? Would we have gone to space and landed on the moon? Or would it have been Germany, with scientists like Werner von Braun? While it may be an afterthought given the rapid rate of change in regards to technology, on this day, the United States was officially drawn into the war that would cement it as a global superpower. December 11th, 1941 Germany declares war on the United States, and history was forever altered.