From Mexico's point of view, the Americans at the Alamo (February 23, 1836 - March 6, 1836) were rebellious foreigners challenging Mexico's sovereignty. On the other hand, the United States justified its westward expansion through its policy of Manifest Destiny. The results of the Alamo shocked the United States and eventually led to the Mexican American War (1846-1848). 

The Treaty of Guadalupe Hidalgo, also known as The Mexican Cession, sealed the American victory in 1848. In return for $15 million and the assumption of Mexican debts to Americans, Mexico relinquished its claims and rights over New Mexico and California. The enormous territory included present-day California, Nevada, Utah, and parts of Arizona, New Mexico, Oklahoma, Colorado and Wyoming. Mexico also agreed to finally relinquish all of Texas, including the disputed area along the border. The U.S. Congress ratified the treaty on March 10, 1848. Mexico, subsequently, ceded additional territory to the United States with the ratification of the Gadsden Purchase.

The Mexican American War, the climatic result of the major tensions that had existed between the two nations, was "one of the most unjust ever waged by a stronger against a weaker nation. It was an instance of a republic following the bad example of European monarchies, in not considering justice in their desire to acquire additional territory." Statement of Union general and Eighteenth President of the United States, Ulysses S. Grant