On June 5, 1942, the United States declared war on Bulgaria. Bulgaria was neutral during 1939–1941, but on March 1, 1941, Bulgaria signed the Tripartite Pact and officially joined the Axis bloc. Following this, the Bulgarian government declared war on the United Kingdom and the United States on December 13, 1941. On June 4, 1942 the United States Congress passed joint resolutions declaring war on Bulgaria along with Hungary and Romania. President Roosevelt approved all three declarations the following day. 
Already one week later, the U.S. bombed the Romanian oilfields at Ploiești. The capital of Bulgaria, Sofia, and other Bulgarian cities, were bombed by Allied aircraft in 1943 and 1944.
As of October 2024[ [update]], the declarations of war against Bulgaria, Hungary, and Romania are the last formal declarations of war by the United States Congress, which in the era of collective security has largely ceded the war power to the President.
Text of the declaration.
JOINT RESOLUTION Declaring that a state of war exists between the Government of Bulgaria and the Government and the people of the United States and making provisions to prosecute the same.
Whereas the Government of Bulgaria has formally declared war against the Government and the people of the United States of America: Therefore, be it
Resolved by the Senate and House of Representatives of the United States of America in Congress assembled, That the state of war between the United States and the Government of Bulgaria which has thus been thrust upon the United States is hereby formally declared; and the President is hereby authorized and directed to employ the entire naval and military forces of the United States and the resources of the Government to carry on war against the Government of Bulgaria; and, to bring the conflict to a successful termination, all the resources of the country are hereby pledged by the Congress of the United States.
References.
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