In 1944, Bulgaria was occupied by the Red Army.

An excerpt from the article 34 facts about Bulgaria

This resulted in the overthrow of the monarchy (the last tsar of Bulgaria was Simeon II) and the establishment of a people's republic in 1946, after which the Bulgarian Communist Party took power in the country. Massive demonstrations in 1989 led to the establishment of a democratic parliamentary republic, and the first free elections were held there on October 13, 1991.