World War II: Great Britain declares war on Finland, Hungary and Romania.

Finland, officially the Republic of Finland, is a Nordic country in Northern Europe. It borders Sweden to the northwest, Norway to the north, and Russia to the east, with the Gulf of Bothnia to the west and the Gulf of Finland to the south, opposite Estonia. Finland has a population of 5.6 million, the majority being ethnic Finns. Its capital and largest city is Helsinki. The official languages are Finnish and Swedish, the mother tongues of 84.1 percent and 5.1 percent of the population, respectively. Finland's climate varies from humid continental in the south to boreal in the north. Its land is predominantly covered by boreal forest, with over 180,000 recorded lakes.

The Kingdom of Hungary was a monarchy in Central Europe that existed for nearly a millennium, from 1000 to 1946 and was a key part of the Habsburg monarchy from 1526-1918. The Catholic kingdom emerged as a continuation of the Grand Principality of Hungary upon the coronation of the first king Stephen I at Esztergom around the year 1000; his family led the monarchy for 300 years. By the 12th century, the kingdom had become a European power.

The Kingdom of Romania was a constitutional monarchy that existed from 25 March [O.S. 13 March] 1881 with the crowning of prince Karl of Hohenzollern-Sigmaringen as King Carol I, until 1947 with the abdication of King Michael I and the Romanian parliament's proclamation of the Romanian People's Republic.

December 5 is the 339th day of the year in the Gregorian calendar; 26 days remain until the end of the year.