Answer: In the end, Texas was admitted to the United States a slave state. The annexation of Texas contributed to the coming of the Mexican-American War (1846-1848). The conflict started, in part, over a disagreement about which river was Mexico's true northern border: the Nueces or the Rio Grande.It stemmed from the annexation of the Republic of Texas by the U.S. in 1845 and from a dispute over whether Texas ended at the Nueces River (the Mexican claim) or the Rio Grande (the U.S. claim).Many Mexicans were unhappy about the annexation of Texas. They feared that annexation was just the first step and that the United States wanted to take over all of Mexico. The conflict between Mexico and the United States was not new.Texas formally joined the union on February 19, 1846. Following the annexation, relations between the United States and Mexico deteriorated because of an unresolved dispute over the border between Texas and Mexico, and the Mexican–American War broke out only a few months later.The annexation of Texas increased tensions with Mexico, because it had never formally recognized Texan independence. The United States and Mexico also disagreed on the location of the southern boundary of Texas. Polk offered Mexico money to settle the dispute and to purchase California and New Mexico.
