In 1845 the United States officially performed an annexation of Texas, at the time Texas was part of Mexico despite the Texas Revolution of 1836. President James K. Polk was pushing American expansionism (Manifest Destiny) interests westward towards California. Leading up to this blatant act of war President Polk had secretly offered Mexico $25 million dollars for the Rio Grande border in Texas and additional Mexican providences Alta California and Santa Fe de Nuevo Mexico1. Government representatives in Mexico City considered this offer an insult, popular opinion within Mexico was that selling these territories and allowing Texas to claim a border as far south as the Rio Grande would tarnish the Mexican national honor2. .
The decline of the purchase offer allowed President Polk to dispatch United States military troops south to the Rio Grande to secure the disputed border. The United States claimed that the border between Texas and Mexico was the Rio Grande, citing the 1836 Treaties of Velasco. Mexico rejected the treaties and refused to negotiate claiming all of Texas3. The Treaties of Velasco were two documents signed at Velasco, Texas (now Freeport, Texas) on May 14, 1836, between Antonio López de Santa Anna of Mexico and the Republic of Texas, in the aftermath of the Battle of San Jacinto (April 21, 1836). The signatories were Interim President David G. Burnet for Texas and General Santa Anna for Mexico. The treaties were intended, on the part of the Texans, to provide a conclusion of hostilities between the two belligerents and offer the first steps toward the official recognition of the breakaway Republic's independence. It set the southern boundary of Texas at the Rio Grande, including the Nueces Strip4. Santa Anna signed both a public treaty and a secret treaty, but neither treaty was ratified by the Mexican government because he had signed the documents under coercion as a prisoner.