Traditional characterizations of the 1846–1848 war between the United States and Mexico emphasize the conventional battles waged between two sovereign nations. However, two little-known guerrilla wars taking place at the same time proved critical to the outcome of the conflict. Using information from twenty-four archives, including the normally closed files of Mexico’s National Defense Archives, Wars Within War breaks new ground by arguing that these other conflicts proved crucial to the course of events. In the first struggle, a force organized by the Mexican army launched a prolonged campaign against the supply lines connecting the port of Veracruz to US forces advancing upon Mexico City. In spite of US efforts to destroy the partisans’ base of support, these armed Mexicans remained a significant threat as late as January 1848. Concurrently, rebellions of class and race erupted among Mexicans, an offshoot of the older struggle between a predominantly criollo elite that claimed European parentage and the indigenous population excluded from participation in the nation’s political and economic life. Many of Mexico’s powerful, propertied citizens were more afraid of their fellow Mexicans than of the invaders from the north. By challenging their rulers, guerrillas forced Mexico’s government to abandon further resistance to the United States, changing the course of the war and Mexican history.