The Mexican-American war began in 1846 and ended in 1848. There were many things that caused this war. Three major causes were the Texas Crisis, Manifest Destiny and the actions of Presidents Tyler and Polk. In 1836 Texas gained independence from Mexico. President John Tyler had recieved enough support to annex Texas. Mexico was upset that Texas would be the 28th state to join the Union. Mexico found this reason enough to call for war. The war began when Polk instigated a fight by moving American troops into a disputed zone between the Rio Grande and the Nueces River. Mexico ended up attacking the American troops which in turn caused The United States to declare war on Mexico on May 13, 1846. The ideal of Manifest Destiny was another cause