<P> During this period, the frontier borderlands to the north became quite isolated from the government in Mexico City, and its monopolistic economic policies caused suffering . With limited trade, the people had difficulty meeting tax payments and resented the central government's actions in collecting customs . Resentment built up from California to Texas . Both the mission system and the presidios had collapsed after the Spanish withdrew from the colony, causing great disruption especially in Alta California and New Mexico . The people in the borderlands had to raise local militias to protect themselves from hostile Native Americans . These areas developed in different directions from the center of the country . </P> <P> Wanting to stabilize and develop the frontier, Mexico encouraged immigration into present - day Texas, as they were unable to persuade people from central Mexico to move into those areas . They allowed for religious freedom for the new settlers, who were primarily Protestant English speakers from the United States . Within several years, the Anglos far outnumbered the Tejano in the area . Itinerant traders traveled through the area, working by free market principles . The Tejano grew more separate from the government and due to its neglect, many supported the idea of independence and joined movements to that end, collaborating with the English - speaking Americans . </P> <P> General Antonio López de Santa Anna, a centralist and two - time dictator, approved the Siete Leyes in 1836, a radical amendment that institutionalized the centralized form of government . When he suspended the 1824 Constitution, civil war spread across the country . Three new governments declared independence: the Republic of Texas, the Republic of the Rio Grande and the Republic of Yucatán . </P> <P> The 1846 United States annexation of the Republic of Texas and subsequent American military incursion into territory that was part of Coahuila (also claimed by Texas) instigated the Mexican--American War . The war was settled in 1848 via the Treaty of Guadalupe Hidalgo . Mexico was forced to give up more than one - third of its land to the U.S., including Alta California, Santa Fe de Nuevo México and the territory claimed by Texas . A much smaller transfer of territory in what is today southern Arizona and southwestern New Mexico--known as the Gadsden Purchase--occurred in 1854 . </P>

Close to 20 of mexico's population lives in and around mexico city making it