Congress has officially declared war 11 times. The first declaration of war was against Britain in 1812 and the most recent was during World War II against Rumania in 1942.
Between 1941 and 1942, war was declared on six countries, including Germany, Japan, Italy, Bulgaria and Hungry. Declarations of war against Germany and Austria-Hungary also occurred in 1917 during World War I. The other two declarations occurred in the 19th century with Spain in 1898 and Mexico in 1846.
Congress has not officially declared war since World War II. Since that time, there have been resolutions and authorizations for the use of military force.