With the Indians of the east subdued, the nation turned its attention to the Mexican holdings of Texas, California, New Mexico, Colorado and other areas that would form the eventual U.S. Southwest. Texas had achieved independence from Mexico in 1836 after the Alamo incident. In 1845, the U.S. annexed Texas and then president James Polk moved troops to the Rio Grande to provoke an incident with with Mexico. U.S. forces were then commanded to forcibly expel Mexicans from the Texas side of the border, despite the fact that the Rio Grande had not traditionally been the dividing line between Southern Mexico and Texas. As the newspapers of the day called for the completion of the Manifest Destiny policy, several members of the of the U.S. Army forces at the “border” came under Mexican attack.
Despite the misgivings of some in the military over the legitimacy of the U.S’s actions, by May of 1846, Polk had secured a declaration of war against Mexico. The war proceeded with popular favor at first, despite some opposition in Congress which included newly-elected House member Abraham Lincoln. Opposition to the war grew as the U.S. advanced deeper into Mexico. Attacks on civilians, destruction of non-military targets and anti-Catholic bigotry led to troop rebellions and defections from the U.S. Army. Further attacks in California by Anglo loyalists also shrunk Mexican territory. After almost two years of warfare, the Treaty of Guadalupe Hidalgo ended the war and seeded all Mexican territory north of the Rio Grande to the United States. The dreams of Manifest Destiny were fulfilled as the United States now stretched from “sea to shining sea.”