What was the result of the Mexican-American War?

The result of the Mexican-American War was an American victory that transferred about 525,000 square miles of territory from Mexico to the United States, including all or part of what would become California, Nevada, Utah, Arizona, New Mexico, Colorado, and Wyoming, redrawing the map of North America and dramatically expanding the United States. The Treaty of Guadalupe Hidalgo signed on February 2, 1848 by American negotiator Nicholas Trist and Mexican commissioners Bernardo Couto, Miguel Atristain, and Luis Cuevas codified the territorial transfer. The United States acquired the Mexican Cession, which together with Texas (annexed in 1845) and Oregon (settled with Britain in 1846) extended American territory from the Atlantic to the Pacific. In return, the United States paid Mexico 15 million dollars and assumed about 3.25 million dollars in claims that American citizens held against the Mexican government.

The Senate ratified the treaty on March 10, 1848 by 38 to 14 with some senators objecting that the United States should have annexed all of Mexico and others objecting to any annexation. Mexico ratified on May 25, 1848. The Gadsden Purchase of December 30, 1853 added 30,000 square miles of southern Arizona and New Mexico for 10 million dollars to provide a southern transcontinental railroad route.

The territorial transfer was vast in scope. California's population, including formerly Mexican citizens who chose American citizenship, would explode after the discovery of gold at Sutter's Mill on January 24, 1848, just nine days before the treaty was signed. About 80,000 Mexican citizens lived in the ceded territory at the time of transfer. They were given a one year window to declare Mexican citizenship and leave or to remain and become American citizens. Most stayed and became American citizens, although their property rights and civil liberties were often violated despite treaty guarantees. Modern descendants of these populations form a significant portion of the Hispanic population of the Southwest.

The war also had political consequences. The Wilmot Proviso introduced in August 1846 and reintroduced repeatedly through 1847 would have banned slavery from any territory acquired from Mexico, but the Senate rejected it. The fight over whether to admit the new territories as slave or free states drove the Compromise of 1850, the Kansas-Nebraska Act of 1854, the Dred Scott decision of 1857, and ultimately the Civil War.

About 13,000 Americans died in the war, mostly from disease, and military costs reached about 100 million dollars. The war launched the political careers of Zachary Taylor, who became President in 1849, and provided combat experience for many Civil War generals including Robert E. Lee, Ulysses S. Grant, William Tecumseh Sherman, Stonewall Jackson, James Longstreet, and George B. McClellan.