On December 11, 1941, in response to Italy declaring war on the United States, four days following the Japanese surprise attack on Pearl Harbor, and three days after the United States declaration of war on the Empire of Japan, the United States Congress passed the Joint Resolution Declaring That a State of War Exists Between The Government of Italy and the Government and the People of the United States and Making Provisions to Prosecute the Same, thereby declaring war against Italy. It also declared war upon Germany that same day.
Text of the declaration
Whereas the Government of Italy 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 Italy 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 Italy; and, to bring the conflict to a successful termination, all of the resources of the country are hereby pledged by the Congress of the United States.