Why were some Americans opposed to annexing Texas?

Many white Texans were eager for Texas to join the United States. Most Texans had originally been American citizens, and they welcomed the protection against Mexico that the U.S. government could provide. Some Americans were eager to add Texas as a new state, but others were deeply opposed. Supporters of the annexation of Texas to the U.S. believed that enlarging the nation was advantageous. Many supporters of annexation also wanted to expand the number of states in which slavery was legal, which would in turn add more pro-slavery votes to the U.S. House of Representatives and Senate.

Other Americans opposed annexation because they feared conflict with Mexico. Opponents believed that the United States should not take land claimed by another nation, especially another republic. More important, they staunchly opposed the spread of slavery. Although some opponents of slavery believed that the U.S. government had no power to interfere with slavery in the Southern states, most believed that it could restrict the spread of slavery into new states and territories. While they did not expect to eliminate slavery in Southern states where it was already legal, they were determined not to allow it to spread into new territories and states in the West.

Controversy between supporters and opponents of annexation made Texas statehood extremely controversial, and the Senate rejected statehood for Texas in 1838 and again in 1844.