Answer:
The last time Congress declared war was in World War II, in December, 1941
(on Japan December 8 and on Germany December 11, 1941).
Declared wars included:
The War of 1812 (1812-1814)
The Mexican War (1846-1848)
The Spanish-American War (1898)
World War I (1917-1919)
World War II (1941-1945)
NOTE
The Korean War and the Vietnam War were not declared wars.
The War in Afghanistan, and both Gulf Wars in Iraq have been undeclared.
Both the US Civil War and Revolutionary War were considered acts of Rebellion.
The Quasi-War with France was un undeclared war in 1800.