Changes

While Mexico did not follow through with its threat to declare war if the United States annexed Texas, relations between the two nations remained tense due to Mexico’s disputed border with Texas. According to the Texans, their state included significant portions of what is today New Mexico and Colorado, and the western and southern portions of Texas itself, which they claimed extended to the Rio Grande River. The Mexicans, however, argued that the border only extended to the Nueces River, north of the Rio Grande.

In July, 1845, Polk, who had been elected on a platform of expansionism, ordered the commander of the U.S. Army in Texas, Zachary Taylor, to move his forces into the disputed lands that lay between the Nueces and Rio Grande rivers. In November, Polk dispatched Congressman John Slidell to Mexico with instructions to negotiate the purchase of the disputed areas along the Texas-Mexican border, and the territory comprising the present-day states of New Mexico and California.