http://www.onwar.com/chrono/1941/dec41/f11dec41.htm
Germany, Italy declare war on US
Thursday, December 11, 1941 www.onwar.com
In Washington... Germany and Italy declare war on the United States. Shortly afterward, the US Congress issues a declaration of war against Germany and Italy.
On the Eastern Front... Soviet advances in the Moscow counteroffensive continue with the recapture of Stalinogorsk.
In North Africa... Rommel’s forces retreating from Tobruk reach Gasala
In the Pacific... Japanese attempts to occupy the island are thrown back. Two Japanese destroyers are also sunk by the 450 US Marines stationed on the island. Relief is promised by Admiral Kimmel.
In Malaya... The 11th Indian Division at Jitra is attacked by advancing Japanese units from Singora.
The true turning point of the war.
There was no rule that the US would have automatically got into the fight against Germany, which had no treaty obligations to come to the aid of Japan in this case. (Not that treaty obligations were much of a concern to Hitler.)
No, Hitler just decided all on his own, and for no particularly logical reason, to declare war on USA. The single act which doomed the Reich. Apparently never even really gave it much thought.