 The political unrest resulting from these losses led to the establishment of a royal authoritarian dictatorship by Tsar Boris III ( 1918 – 1943 ) . Bulgaria entered World War II in 1941 as a member of the Axis but declined to participate in Operation Barbarossa and saved its Jewish population from deportation to concentration camps . The sudden death of Boris III in the summer of 1943 pushed the country into political turmoil as the war turned against Germany and the communist guerrilla movement gained momentum . The government of Bogdan Filov subsequently failed to achieve peace with the Allies . Bulgaria did not comply with Soviet demands to expel German forces from its territory , resulting in a declaration of war and an invasion by the USSR in September 1944 . The communist @-@ dominated Fatherland Front took power , ended participation in the Axis and joined the Allied side until the war ended . 
