What was the last conflict for which the U.S. Congress issued a declaration of war?

The last conflict for which the U.S. Congress issued a formal declaration of war was World War II. This is significant because it marks the end of a specific era in which declarations of war were the primary means by which the U.S. government authorized military action. Both the Vietnam War and the Korean War were engaging periods for U.S. military involvement; however, they did not involve formal declarations of war by Congress. Instead, these conflicts were authorized through various resolutions and other legislative measures. World War I, although also formally declared, occurred before World War II and did not represent the most recent instance. The formal declaration of war during World War II included Japan, Germany, and Italy, and these declarations set a precedent in mobilizing national resources and civilian support for the war effort, thus solidifying its importance in American military history.