The history of Bulgaria during World War II encompasses an initial period of neutrality until 1 March 1941, a period of alliance with the Axis Powers until 8 September 1944, and a period of alignment with the Allies in the final year of the war. 

Bulgarian military forces occupied with German consent parts of the kingdoms of Greece and Yugoslavia which Bulgarian irredentism claimed on the basis of the 1878 Treaty of San Stefano. 

Bulgaria resisted Axis pressure to join the war against the Soviet Union, which began on 22 June 1941, but did declare war on Britain and the United States on 13 December 1941. 

The Red Army entered Bulgaria on 8 September 1944; Bulgaria declared war on Germany the next day.

As an ally of Nazi Germany, Bulgaria participated in the Holocaust, causing the deaths of 11,343 Jews, and though 48,000 Jews survived the war, they were subjected to forcible internal deportation, dispossession, and discrimination. 

Bulgaria functioned as an authoritarian state during most of World War II. 

Tsar Boris III (reigned 1918–1943) ruled through a parliament and a prime minister. Bulgaria’s wartime government was pro-German under Georgi Kyoseivanov, Bogdan Filov, Dobri Bozhilov, and Ivan Bagryanov. It joined the Allies under Konstantin Muraviev in early September 1944, then underwent a coup d’état a week later, and under Kimon Georgiev was pro-Soviet thereafter.