The Mexican–American War, also known as the First American Intervention, the Mexican War, or the U.S.–Mexican War, was an armed conflict between the United States of America and Mexico from 1846 to 1848 in the wake of the 1845 U.S. annexation of Texas, which Mexico considered part of its territory despite the 1836 Texas Revolution.

Combat operations lasted a year and a half, from spring 1846 to fall 1847. American forces quickly occupied New Mexico and California, then invaded parts of Northeastern Mexico and Northwest Mexico; meanwhile, the Pacific Squadron conducted a blockade, and took control of several garrisons on the Pacific coast further south in Baja California. After Mexico would still not agree to the cession of its northern territories, another American army captured Mexico City, and the war ended in victory of the U.S.

The Treaty of Guadalupe Hidalgo specified the major consequence of the war: the forced Mexican Cession of the territories of Alta California and New Mexico to the U.S. in exchange for $18 million. In addition, the United States forgave debt owed by the Mexican government to U.S. citizens. Mexico accepted the Rio Grande as its national border, and the loss of Texas.

Mexican Americans are Americans of Mexican descent. As of July 2009, Mexican Americans make up 10.3% of the United States' population with over 31,689,000 Americans listed as of Mexican ancestry. Mexican Americans comprise 66% of all Hispanics and Latinos in the United States. The United States is home to the second largest Mexican community in the world second only to Mexico itself comprising nearly 22% of the entire Mexican origin population of the world. Canada is a distant third with a Mexican origin population of 37,000 as of 2001 although increasing to 61,505 as of 2006. In addition, as of 2008 there were approximately 7,000,000 undocumented Mexicans living in the United States which if included in the count would increase the US share to over 28% of the world's Mexican origin population (Note that some of the undocumented would be captured in the US Census count depending on their willingness to provide information). Over 60% of all Mexican Americans reside in the states of California and Texas. Most Mexican Americans are the descendants of the Indigenous peoples of Mexico and/or Europeans, especially Spaniards.