President Polk had a strong belief in the “manifest destiny”, the belief that the expansion of the U.S. across America was necessary. In 1836, Texas had gained its independence from Mexico, however, because of opposing political beliefs on adding another slave state, it was not included into the Union. Border raids were encouraged by the Mexican government as well as threatening to initiate war if annexation was attempted. Polk believed that Texas should be re-incorporated, and Oregon should be re-occupied, and focused on gaining California and New Mexico. When his offer to purchase those lands were rejected, he instigated a fight by moving troops into a zone between the Rio Grande and Nueces River
Feb. 2, 1848, the Treaty of Guadalupe Hidalgo was signed, establishing the Rio Grande and not the Nueces River as the U.S.-Mexican border. Under the treaty, Mexico also recognized the U.S. annexation of Texas, and agreed to sell California and the rest of its territory North of the Rio Grande for $15 million plus the assumption of certain damages claims.