Mexico controlled the territory later known as the Mexican Cession, with considerable local autonomy punctuated by several revolts and few troops sent from central Mexico, in the period from 1821–22 after independence from Spain up through 1846 when U.S. military forces seized control of California and New Mexico on the outbreak of the Mexico - Ethnic groups | Britannica Mexico’s population is composed of many ethnic groups, including indigenous American Indians (Amerindians), who account for less than one-tenth of the total. Generally speaking, the mixture of indigenous and European peoples has produced the largest segment of …