The Mexican-American War, also known as the Mexican War in the United States and the Intervención estadounidense en México (United States intervention in Mexico) in Mexico, was an armed conflict from 1846 to 1848 between the United States and Mexico. It stemmed from the annexation of Texas by the United States in 1845 and from a dispute over the southern border of Texas. 

The Mexican-American War began on April 25, 1846, when American troops crossed the Rio Grande and invaded Mexican territory, and ended on February 2, 1848, with the signing of the Treaty of Guadalupe Hidalgo. The treaty resulted in Mexico ceding nearly all of its northern territory, including present-day California, Arizona, New Mexico, and parts of Colorado and Texas, to the United States.

The Mexican-American War is one of the most controversial and least understood wars in American history. It lasted two years, from 1846 to 1848, and resulted in the United States victory and Mexico's cession of nearly half of its territory, including present-day California, Utah, Nevada, and parts of Colorado, New Mexico, and Arizona.

The war was principally battled over the issue of slavery. The United States abolished slavery in 1808, but it still existed in the Mexican province of Texas, which had been part of the Spanish Empire until 1821. In 1836, Texas declared independence and became a republic. In 1845, the United States annexed Texas, and Mexico responded by severing diplomatic relations.

In May 1846, Mexican troops crossed the Rio Grande and attacked a U.S. Army unit in present-day Texas. This event, known as the 'Bear Flag Revolt,' led to the outbreak of war.

The United States quickly invaded Mexico, winning a series of decisive battles. The Mexican capital of Mexico City was captured by US troops in September 1847. The war ended with the Treaty of Guadalupe Hidalgo, which was signed on February 2, 1848. Under the treaty, Mexico ceded nearly half of its territory to the United States.

The war had far-reaching consequences for both countries. In Mexico, it led to the overthrow of the government and to a period of instability and violence that lasted for decades. In the United States, the war resulted in the expansion of territory and the acquisition of a vast new empire. 

The Mexican-American War was one of the most important events in American history. It led to the expansion of the United States and the creation of the American West. It also had a lasting impact on Mexico, shaping the country's national identity and sparking a wave of emigration to the United States.