The Thornton Affair represented a pivotal moment in the escalating tensions between the United States and Mexico in the mid-19th century. The backdrop of this conflict lay in the U.S. annexation of Texas in 1845, a move not recognized by Mexico, which still considered Texas a rebellious province. As U.S. forces moved into the contested areas, they provoked increasing irritation among Mexican officials and military personnel.

On April 25, 1846, Captain Seth Thornton led a reconnaissance mission of around 60 U.S. soldiers into a disputed area between the Rio Grande and the Nueces River. While patrolling the territory, they were ambushed by a Mexican cavalry contingent, leading to a fierce skirmish. The encounter resulted in the deaths of 16 U.S. soldiers and wounded many others. This clash not only inflamed passions on both sides, but also solidified public opinion in the United States for a broader military engagement.

In the wake of the Thornton Affair, President James K. Polk described the incident as an act of invasion by Mexico and sought congressional approval for war. The House of Representatives quickly passed a resolution declaring that a state of war existed between the two nations, which legislated the beginning of the Mexican-American War. This war would ultimately have significant territorial consequences, with the U.S. gaining much of present-day California, Arizona, New Mexico, and other territories as a result of the conflict.