Bulgaria signed the Axis Pact on March 1, 1941. Her troops fought limited naval engagements on the Eastern Front but did not send forces to Operation Barbarossa.
Bulgaria declared war on the United States on December 11, 1941, with Nazi Germany. American aircraft bombed Sofia. Bulgarian Air Forces engaged United States Army Air Force bombers transiting to Ploesti, Romania, to bomb the oil refinery there. Royal Air Force bombers were also engaged. Crews that were shot down were interred for the duration of the war.
In 1944, the Soviet Red Army invaded Bulgaria. Bulgaria declared neutrality, leaving the Axis, but Soviet Premier Josef Stalin indicated he would not respect it, since the Bulgarian government gave the Nazis 8,000 railway wagons to withdraw. The “Fatherland Front,” a nationalist Bulgarian movement, rose up against the government, and a new government was formed — without the Fatherland Front. They called for strikes. To give time for Bulgarian troops to leave occupied Macedonia, the new government withheld declaring war on Germany. So the Red Army declared war on Bulgaria on September 8, 1944. Bulgaria was simultaneously at war with Germany, the United Kingdom, the Soviet Union and the United States.
The next day the Fatherland Front took control in Sofia, immediately declared war on Germany, and began to seek an armistice with the Allies. This left some Bulgarians in Yugoslavia stranded — surrounded unexpectedly by the Nazis. Many fought their way back across the old prewar demarcation of the Bulgarian border. The Bulgarian First Army, one of three with the Soviets, joined with the British Eighth Army in Austria in May 1945.
An estimated 10,000 Bulgarians died in World War II.