The Mexican American War started in 1846. But several years before the war in 1834, there were over 20,000 Americans living in Texas and that greatly outnumber the Tejanos(people of Spanish descent). This is a key factor in the war because on March 2, 1836, Texas declared its independence from Mexico. The main causes for this war was James Polk's desire to expand America, the annexation of Texas to the U.S., the ongoing conflict between the U.S. and Mexico, an when Mexican troops attacked American soldiers and killed them on American soil. This made James Polk furious and America declared war on Mexico immediately after that. The most famous battle of the Mexican American War is the Alamo. Only 189 Texans fought against 5,000 Mexican soldiers ans every one of the Texans were killed except for the few women and children that were spared. America won the next battle though which ended the war with another American victory. during the last battle the Texans chanted "Remember the Alamo!" The Treaty of Guadalupe Hidalgo forced Mexico to give up about half of its territory. The U.S. agreed to pay Mexico $15 million and to protect the Mexicans living in the Mexican Cession(the U.S. didn't live up to this promise). Later on the Gadsden Purchase was made and it gave a small amount of land to the U.S. that was flat and very good for building railroad routes.