In 1844 the United States presidential election was against James Knox Polk and Henry Clay. The Democratic Party supported Polk and the Whig Party supported Clay. The Democratic Party was for Manifest Destiny and for the annexation of Texas; most farmers supported this political party. The Whig party was against annexation because Texas would come into the Union as a slave state and annexation could cause problems with Mexico. Most of the voters however were for annexation and James Polk won the election.

Before the election of 1844 the Treaty of annexation failed. In this treaty of annexation Texas would come into the union as a territory and not a state. Texas would also have to give up some of their land. The Treaty of Annexation failed because they could not get two thirds vote of the senate to approve it. Polk was able to get Texas annexed by a joint resolution. In a joint resolution a simple majority of congress would have to approve it. This was successful and on December 29th 1845 Texas entered the Union has the 28th and last slave state. Texas was able to keep its public land and skip the territorial stage.

On February 19, 1846 Anson Jones the last president of the Republic of Texas turned over the government to James Pinckney Henderson, the first governor of the state of Texas. The first two senators of Texas were Sam Houston and Thomas Rusk. The annexation of Texas caused a boundary dispute between Mexico and the United States. The United States wanted the Rio Grande River to separate the two countries and Mexico wanted the Nueces, a river further north. The diplomats could not solve the boundary dispute and this became one cause of the Mexican War, which was fought from 1846 through 1848.

The United States won the Mexican War and with the signing of the Treaty of Guadalupe Hidalgo on February 2, 1848, the United States received the Mexican Cession and the Rio Grande River became the official border river of the two countries. Mexico was paid 15 million dollars for this land. In 1850, the Pearce Act, which was part of the compromise of 1850, settled the boundary dispute of the Rio Grande between New Mexico and Texas. The Federal government paid Texas 10 million dollars and Texas gave up the disputed land to New Mexico. The western boundary of Texas was now established.