On December 13, 1941, Bulgaria declared war on the USA and Great Britain. It lasted until October 28, 1944 and led to the coup of September 9, 1944. This was required by our allied obligations to the Third Reich.
Bulgaria has no obligation to declare war on the Atlantic allies according to the clauses of the Tripartite Pact, because the USA nominally finds itself in the role of a victim after the attack on Pearl Harbor.
On December 11, 1941, Germany declared war on the USA, using as an excuse several attacks on German submarines and merchant ships carried out by the American Atlantic Fleet. They were carried out on the orders of US President Roosevelt. On December 12, Germany was followed by Italy, which also declared war on the United States. They were joined in the following days by Romania, Hungary and Bulgaria.
On June 5, 1942, the United States also declared war on the Kingdom of Bulgaria. On June 12, 1942, 13 long-range B-24 Liberator heavy bombers, based at Fayd Airport on the Suez Canal, flew over the Mediterranean Sea, Turkey and the Black Sea, passed over land in the area of Constanța and bombed the oil region of Ploiești. Damage to the refineries was minor. The air defense and fighter aircraft of the Luftwaffe and the Romanian Air Force did not react. This was the first strike by American strategic aviation deep into the continent.
In the fall of 1943, as a result of the successful Allied landing in Sicily, the territory of Southern Italy brought the Balkans within the reach of Allied aviation. The bombing of Bulgaria began, lasting from October 20, 1943 to September 17, 1944.
Cities in the annexed parts of Yugoslavia were also bombed, after prior coordination with Tito.