December 3: Fighting breaks out between Communists and royalists in newly liberated Greece, eventually leading to a full-scale Greek Civil War. - The British Home Guard is stood down.
December 7: Convention on International Civil Aviation signed in Chicago to create the International Civil Aviation Organization.
December 10: Legendary Italian conductor Arturo Toscanini leads a concert performance of the first half of Beethoven's Fidelio (minus its spoken dialogue) on NBC Radio, starring Rose Bampton. He chooses this opera for its political message - a statement against tyranny and dictatorship. Conducting it in German, Toscanini intends it as a tribute to the German people who are being oppressed by Hitler. The second half is broadcast a week later. The performance is later released on LP and CD, the first of 7 operas that Toscanini conducts on radio.
December 12-December 13: British units attempt to take the hilltop town of Tossignano, but are repulsed.
December 13: Battle of Mindoro: United States, Australian and Philippine Commonwealth troops land on Mindoro Island in the Philippines.
December 14: The Soviet government changes Turkish place names to Russian in the Crimea. - United States release of the film National Velvet which brings a young Elizabeth Taylor to stardom.
December 15: A private airplane carrying bandleader Glenn Miller disappears in heavy fog over the English Channel while flying to Paris.
December 16: Germany begins the Ardennes offensive, later known as Battle of the Bulge. - General George C. Marshall becomes the first U.S. Five-Star General.
December 17: Malmedy massacre: German SS troops under Joachim Peiper machine gun American prisoners of war captured during the Battle of the Bulge near Malmedy and elsewhere in Belgium.
December 19: The entire territory of Estonia is taken by the Red Army.
December 20: The U.S. Women Airforce Service Pilots are disbanded.
December 22: Brigadier General Anthony C. McAuliffe, commander of the U.S. forces defending Bastogne, refuses to accept demands for surrender by sending a one-word reply, "Nuts!", to the German command. - The Vietnam People's Army is formed in Vietnam.
December 24: The Bulge reaches its deepest point at Celles. - Troopship SS Leopoldville is sunk in the English Channel by German submarine U-486. 763 soldiers of the U.S. 66th Infantry Division bound for the Battle of the Bulge drown. - The first complete U.S. production of Tchaikovsky's ballet The Nutcracker is presented in San Francisco, choreographed by William Christensen. It will become an annual tradition there, and for the next ten years, the San Francisco Ballet will be the only company in the United States performing the complete work.
December 26: American troops repulse German forces at Bastogne. - The original stage version of The Glass Menagerie by Tennessee Williams premieres on Broadway.
December 30: Edward Stettinius, Jr., becomes the last United States Secretary of State of the Roosevelt administration, filling the seat left by Cordell Hull. - King George II of Greece declares a regency, leaving his throne vacant. - "Stage Door Cartoon" was the first cartoon to be produced by Eddie Selzer.
December 31: Hungary declares war on Nazi Germany. - Battle of Leyte: Tens of thousands of Imperial Japanese Army soldiers are killed in action, in a significant Filipino and Allied military victory. (source: Wikipedia)