James K. Polk Delivers A Message To Congress

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). 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.” A joint session of Congress approved the declaration of war, with southern Democrats in strong support. Sixty-seven Whigs voted against the war on a key slavery amendment, but on the final passage only 14 Whigs voted no, 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.

The Thornton Affair, also known as the Thornton Skirmish, was an incident between the military forces of the United States and Mexico. It served as the primary justification for U.S. President James K. Polk's declaration of war against Mexico in 1846, sparking the Mexican-American War. Now days, the affair is vastly overlooked, with most people considering the Battle of Palo Alto as the first engagement in the war.