The overall outcome of the Mexican - American War (1846-1848) was determined largely by regular military forces. There was, however, another war within this war waged by irregulars, guerrillas, and rancheros from Mexico, as well as by Texas Rangers and elements within the U.S. volunteers. Although these forces were important to both sides in the war effort, people most often remember them for the atrocities they committed.
Mills, J. W. (2009). Irregulars: Guerrilla and Ranchero Warfare in South Texas and Northern Mexico during the Mexican - American War. Journal of South Texas, 22(1), 1–20.
Journal of South Texas