When did the US Congress last declare war on another country?
The US Congress has formally declared war only 11 times in U.S. history.
The first declaration of war was on Great Britain in 1812. Also called the "second war for independence", this conflict lasted for two and a half years and ended with the Treaty of Ghent, ratified by the Senate on the 17th of February, 1815. The end of this war was the beginning of the peaceful American-British relationships .
The 11th and last declaration was on Rumania in June 1942, during WW2. Since World War II the Congress hasn't officially declared war on any country of the world.