According to the U.S. Senate Website the last war declared was on June 4th, 1942. The war was declared against Rumania.
In total, congress has declared war 11 times.
The Constitution of the United States puts the power to declare war solely in the hands of Congress. This power is found in Article I, Section 8. It states: “The Congress shall have Power…To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water.”
According to the Senate website, since the last declaration of war, resolutions have been used to authorize the use of military force.