The government of the Kingdom of Bulgaria under Prime Minister Bogdan Filov declared a position of neutrality upon the outbreak of World War II.
Bulgaria was determined to observe it until the end of the war; as it hoped for bloodless territorial gains, especially in the lands with a significant Bulgarian population occupied by neighbouring countries after the Second Balkan War and World War I.
However, it was clear that the geopolitical position of Bulgaria in the Balkans would inevitably lead to strong external pressure by both World War II
factions. Bulgaria joined the Axis Powers in 1941, when German troops preparing to invade Yugoslavia and Greece reached the Bulgarian borders and demanded permission to pass through Bulgarian territory. On
the 1st of March 1941, Bulgaria signed the Tripartite Pact and officially joined the Axis bloc. With the Soviet Union in a non-aggression pact with Germany, there was little popular opposition to the
decision. Bulgaria did not join the German invasion of the Soviet Union that began on
the 22nd of June 1941 nor did it declare war on the Soviet Union. However, despite the lack of official declarations of war by both sides, the Bulgarian Navy was involved in a number of skirmishes with the Soviet Black Sea Fleet, which attacked Bulgarian shipping. Besides this, Bulgarian armed forces garrisoned in the Balkans battled various resistance groups.
The Bulgarian government was forced by Germany to declare a token war on the United Kingdom and the United States on 13 December 1941, an act which resulted in the bombing of Sofia and other Bulgarian cities by Allied aircraft.
Bulgaria had maintained diplomatic relations with the Soviet Union while being a member of the Axis Powers. In the summer of 1944, after having crushed the Nazi defence around Iaşi and Chişinău, the Soviet Army was approaching the Balkans and Bulgaria. On
the 23rd of August 1944, Romania left the Axis Powers and declared war on Germany, and allowed Soviet forces to cross its territory to reach Bulgaria. On
the 5th of September, the Soviet Union declared war on Bulgaria and invaded. Within three days, the Soviets occupied the northeastern part of Bulgaria along with the key port cities of Varna and Burgas. The Bulgarian Army was ordered to offer no resistance. On
the 8th of September, the Bulgarians changed sides and joined the Soviet Union in its war against Nazi Germany.
The Bulgarian Air Force at the outset of war comprised of just 91
aircraft mainly obsolete.