Texas declared its independence from Mexico in 1836 after the Texas Revolution. It became the Republic of Texas. Even as Texas declared its independence, Mexico continued to claim Texas as its territory. The Republic of Texas claimed as its boundary the Rio Grande. While at the same time, Tamaulipas, a northern Mexican state, claimed area north of the Rio Grande to the Nueces River.

The annexation of Texas had been debated in the U.S. for several years. Antislavery forces feared the Texas territory could become a new slave state. President John Tyler was interested in securing the Texas territory because the British were interested in the territory. Texas was finally annexed by the U.S. in February 1845.

Mexico’s government was very unstable; they had a number of different leaders. In 1843, the Mexican government told the United States that it would "consider equivalent to a declaration of war . . . the passage of an act for the incorporation of Texas."

President James Polk ordered Brigadier General Zachary Taylor and the Army of Occupation to the mouth of the Rio Grande to defend Texas from the Mexicans. General Taylor arrived in March of 1846.