On December 11, 1941, in response to the Italian declaration of 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. Wikipedia