Only four days after the December 7, 1941 Japanese attack on Pearl Harbor, the United States (which had managed to remain more or less neutral as the conflicts of WWII spread across Europe and parts of Asia) found itself pitted against yet another Axis power – Germany. Hitler had decided to take action against the United States.
On December 11, 1941, in front of a packed theater of his Reichstag peers (members of the German state legislature), Hitler delivered the speech that would change the course of WWII and solidify Germany's eventual defeat. By choosing to pursue aggressions with the United States, Hitler ensured a continued alliance between Britain and the US, severely underestimating the extent to which the United States was militarily and industrially prepared to fight overseas.
Hitler's decision has been viewed as rash and ill advised, and it has even been attributed to his coming out of hurt ego, which got bruised when his Japanese allies went after the US without consulting him and he wanted to regain an upper hand in the strategically delicate relationship. Regardless of his reasoning, Hitler's speech that afternoon was one of impassioned rhetoric and devastating consequences for the future of the Third Reich.