San Felipe de Neri built in 1793 is the oldest church in Old Town Albuquerque, New Mexico and also the oldest in New Mexico. Many Americans forget that the whole Southwestern United States was once owned by Mexico. Most of the territory was obtained through the Mexican War of 1846 to 1848. One of the ironies of the War was that some of the generals and other officers fighting for the U.S. would soon be fighting against the United States in the Civil War. 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. Arizona later obtained additional parts of Mexico through the Gadsen Purchase.