Thornton Affair

The Thornton Affair, also known as the Thornton Skirmish, Thornton's Defeat, or Rancho Carricitos was a battle in 1846 between the military forces of the United States and Mexico 20 miles west upriver from Zachary Taylor's camp along the Rio Grande. The much larger Mexican force defeated the Americans in the opening of hostilities, and was the primary justification for U.S. President James K. Polk's call to Congress to declare war.
Although the United States had annexed Texas, both the US and Mexico claimed the area between the Nueces River and the Rio Grande. Polk had ordered Taylor's Army of Occupation to the Rio Grande early in 1846 soon after Mexican President Mariano Paredes declared in his inaugural address to uphold the integrity of Mexican territory to the Sabine River.

Wikipedia