Declaration Of War

By then, Polk had received word of the Thornton Affair. This, added to the Mexican government's rejection of Slidell, Polk believed, constituted a casus belli (case for war).[15] His message to Congress on May 11, 1846 stated that “Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon American soil.”[16][17] Congress approved the declaration of war on May 13, with southern Democrats in strong support. Sixty-seven Whigs voted against the war on a key slavery amendment,[18] but on the final passage only 14 Whigs voted no,[18] including Rep. John Quincy Adams. Congress declared war on Mexico on May 13, 1846 after only having a few hours to debate. Although President Paredes's issuance of a manifesto on May 23 is sometimes considered the declaration of war, Mexico officially declared war by Congress on July 7.