December 1: Fiorello La Guardia, Mayor of New York City and Director of the Office of Civilian Defense, signs Administrative Order 9, creating the Civil Air Patrol under the authority of the United States Army Air Forces. - A state of emergency is declared in British Malaya and the Straits Settlements.
December 2: Attack on Pearl Harbor: The code message "Climb Mount Niitaka" is transmitted to the Japanese task force, indicating that negotiations have broken down and that the attack is to be carried out according to plan.
December 4: The State of Jefferson is declared in Yreka, California, with judge John Childs as a governor.
December 6: Soviet counterattacks begin against German troops encircling Moscow. The Wehrmacht is subsequently pushed back over 200 miles. - The United Kingdom declares war on Finland.
December 6: British submarine HMS Perseus is sunk by a mine off Cephalonia.
December 7 (December 8 3:18 a.m., Japan Standard Time) - - Attack on Pearl Harbor: Aircraft flying from Imperial Japanese Navy carriers launch a surprise attack on the United States fleet at Pearl Harbor in Hawaii, thus drawing the United States into World War II. The attack begins at 7:48 a.m. local time and is announced on radio stations in the U.S. at about 2:26 p.m. EST (19.26 GMT). - The Japanese declaration of war on the United States and the British Empire is published in Japanese evening newspapers but not formally delivered to the U.S. until the following day. Canada declares war on Japan. - Tobruk's British and Commonwealth garrison is relieved after Axis forces under Rommel withdraw.
December 8: The Battle of Hong Kong begins shortly after 8:00 a.m. (local time), less than eight hours after the attack on Pearl Harbor, when Japanese forces invade Hong Kong, which is defended by British, Canadian and local troops. The United Kingdom officially declares war on the Empire of Japan. - The Japanese occupation of the Philippines begins ten hours after the attack on Pearl Harbor when Japanese forces invade Luzon and destroy U.S. aircraft on Clark Field. - President of the United States Franklin D. Roosevelt delivers his "Infamy Speech" to a Joint session of the United States Congress at 12:30 p.m. EST (17.30 GMT). Transmitted live over all four major national networks it attracts the largest audience ever for an American radio broadcast, over 81% of homes. Within an hour, Congress agrees to the President's request for a United States declaration of war upon Japan and he signs it at 4:10 p.m. - Australia, New Zealand, The Netherlands, the Free French, Yugoslavia, Costa Rica, Cuba, El Salvador and Honduras also officially declare war on Japan, and the Republic of China declares war on the Axis powers. - Japanese also attack British Malaya and Thailand. - Guatemala declare war on Japan. - The German advance on Moscow (Operation Typhoon) is suspended for the winter. - the Nazi German Che?mno extermination camp opens in occupied Poland near the village of Che?mno nad Nerem. Between December 1941-April 1943 and June 1944-January 1945 at least 153,000 people will be killed in the camp.
December 10: The British battleship HMS Prince of Wales and battlecruiser HMS Repulse are sunk by Japanese aircraft in the South China Sea north of Singapore.
December 11: Germany and Italy declare war on the United States. The U.S. responds in kind. - Mildred Gillars ("Axis Sally") delivers her first propaganda broadcast to Allied troops.
December 12: Hungary and Romania declare war on the United States. - British India declares war on the Empire of Japan. - The United States seizes the French ship SS Normandie. - The Kimura Detachment of the Japanese Imperial forces is occupied in Legaspi, Albay in Eastern Philippines.
December 13: Sweden's low temperature record of -53ºC is set in a village within the Vilhelmina Municipality.
December 19: Hitler becomes Supreme Commander-in-Chief of the German Army.
December 22: Arcadia Conference opens in Washington, D.C., the first meeting on military strategy between the heads of government of the United Kingdom and the United States following the latter's entry into the war.
December 23: A second Japanese landing attempt on Wake Island is successful, and the American garrison surrenders after a full night and morning of fighting.
December 24: British forces capture Benghazi. - Dutch submarine HNLMS K XVI is the first Allied ship to sink a Japanese warship, sinking the destroyer Sagiri near Sarawak; K XVI is herself torpedoed the following day by Japanese submarine I 66.
December 25: The Battle of Hong Kong ends after 17 days with surrender of the British Crown colony to the Japanese. - Admiral Emile Muselier seizes the archipelago of Saint Pierre and Miquelon, the first part of France to be liberated by the Free French Forces.
December 26: Winston Churchill becomes the first British Prime Minister to address a joint session of the United States Congress.
December 27: British Commandos raid the Norwegian port of Vaagso, causing Hitler to reinforce the garrison and defenses, drawing vital troops away from other areas. (source: Wikipedia)