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 (32 km) 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.

The Centralist Republic of Mexico, or in the anglophone scholarship, the Central Republic, officially the Mexican Republic, was a unitary political regime established in Mexico on 23 October 1835, under a new constitution known as the Siete Leyes after conservatives repealed the federalist Constitution of 1824 and ended the First Mexican Republic. It would ultimately last until 1846, when the Constitution of 1824 was restored at the beginning of the Mexican-American War.