Roots of the conflict in Northern Mexico
Comanches of West
in war regalia, c. 1830
The 1832 boundaries of
, the Comanche homeland
The northern area of
Mexico was sparsely settled and not well controlled politically by the government based in Mexico City. After
Spain in 1821, Mexico contended with internal struggles that sometimes verged on civil war and the northern frontier was not a high priority. In the sparsely settled interior of northern Mexico, the end of Spanish rule was marked by the end of financing for
presidios and for subsidies to
indigenous Americans to maintain the peace. There were conflicts between indigenous people in the northern region as well. The
Comanche were particularly successful in expanding their territory in the
Comanche–Mexico Wars and garnering resources. The
Apache–Mexico Wars also made Mexico's north a violent place, with no effective political control.
Apache raids left thousands of people dead throughout northern Mexico. When the
United States Army entered northern Mexico in 1846 they found demoralized Mexican settlers. There was little resistance to US forces from the civilian population.
Hostile activity from indigenous people also made communications and trade between the interior of Mexico and provinces such as
Alta California and
New Mexico difficult. As a result, New Mexico was dependent on the overland
Santa Fe Trail trade with the
United States at the outbreak of the Mexican–American War.
Mexico's military and diplomatic capabilities declined after it attained independence and left the northern half of the country vulnerable to the Comanche, Apache, and
Navajo. The indigenous people, especially the Comanche, took advantage of the weakness of the Mexican state to undertake large-scale raids hundreds of miles into the country to acquire livestock for their own use and to supply an expanding market in
Texas and the US.
The Mexican government's policy of settlement of US citizens in
its province of Tejas was aimed at expanding control into Comanche lands, the
Comancheria. Instead of settlement occurring in the central and west of the province, people settled in
East Texas, where there was rich farmland and which was contiguous to southern US
slave states. As settlers poured in from the US, the Mexican government took steps to discourage further settlement, including its 1829 abolition of slavery.
In 1836, Mexico was relatively united in refusing to recognize the
independence of Texas. Mexico threatened war with the United States if it annexed the
Republic of Texas.
 Meanwhile, U.S. President Polk's assertion of
Manifest Destiny was focusing United States interest on westward expansion beyond its existing national borders.
Designs on California
During the Spanish colonial era, the Californias (i.e., the Baja California peninsula and Alta California) were sparsely settled. After Mexico became independent, it shut down the missions and reduced its military presence.
In 1842, the US minister in Mexico,
Waddy Thompson Jr., suggested Mexico might be willing to cede Alta California to settle debts, saying: "As to Texas, I regard it as of very little value compared with California, the richest, the most beautiful, and the healthiest country in the world ... with the acquisition of Upper California we should have the same ascendency on the Pacific ... France and England both have had their eyes upon it."
John Tyler's administration suggested a tripartite pact that would settle the
Oregon boundary dispute and provide for the cession of the port of San Francisco from Mexico;
Lord Aberdeen declined to participate but said Britain had no objection to U.S. territorial acquisition there.
The British minister in Mexico,
Richard Pakenham, wrote in 1841 to
Lord Palmerston urging "to establish an English population in the magnificent Territory of Upper California", saying that "no part of the World offering greater natural advantages for the establishment of an English colony ... by all means desirable ... that California, once ceasing to belong to Mexico, should not fall into the hands of any power but England ... daring and adventurous speculators in the United States have already turned their thoughts in this direction." But by the time the letter reached London, Sir
Robert Peel's Tory government, with its
Little England policy, had come to power and rejected the proposal as expensive and a potential source of conflict.
A significant number of influential
Californios were in favor of annexation, either by the
United States or by the
Pío de Jesús Pico IV, the last governor of Alta California, was in favor of British annexation.
Republic of Texas
Republic of Texas
. The present-day outlines of the individual U.S. states are superimposed on the boundaries of 1836–1845.
In 1800, the colonial province of Texas was sparsely populated, with only about 7,000 non-Indian settlers.
 The Spanish crown developed a policy of colonization to more effectively control the territory. After independence, the Mexican government implemented the policy, granting
Moses Austin, a banker from Missouri, a large tract of land in Texas. Austin died before he could bring his plan of recruiting American settlers for the land to fruition, but his son,
Stephen F. Austin, brought over 300 American families into Texas.
 This started the steady trend of migration from the United States into the Texas frontier. Austin's colony was the most successful of several colonies authorized by the Mexican government. The Mexican government intended the new settlers to act as a buffer between the
Tejano residents and the
Comanches, but the non-Hispanic colonists tended to settle where there was decent farmland and trade connections with American Louisiana, which the United States had acquired in the
Louisiana Purchase, rather than further west where they would have been an effective buffer against the Indians.
In 1829, as a result of the large influx of American immigrants, the non-Hispanic outnumbered native Spanish speakers in the Texas territory. President
Vicente Guerrero, a hero of Mexican independence, moved to gain more control over Texas and its influx of southern non-Hispanic colonists and discourage further immigration by abolishing slavery in Mexico.
 The Mexican government also decided to reinstate the property tax and increase tariffs on shipped American goods. The settlers and many Mexican businessmen in the region rejected the demands, which led to Mexico closing Texas to additional immigration, which continued from the United States into Texas illegally.
In 1834, General
Antonio López de Santa Anna became the centralist dictator of Mexico, abandoning the federal system. He decided to quash the semi-independence of Texas, having succeeded in doing so in Coahuila (in 1824, Mexico had merged Texas and Coahuila into the enormous state of
Coahuila y Tejas). Finally,
Stephen F. Austin called Texians to arms; they declared independence from Mexico in 1836, and after Santa Anna
defeated the Texians at the Alamo, he was defeated by the
Texian Army commanded by General
Sam Houston and captured at the
Battle of San Jacinto and signed a treaty recognizing Texas' independence.
Texas consolidated its status as an independent republic and received official recognition from Britain, France, and the United States, which all advised Mexico not to try to reconquer the new nation. Most
Texians wanted to join the United States of America, but annexation of Texas was contentious in the
US Congress, where Whigs were largely opposed. In 1845 Texas agreed to the offer of annexation by the US Congress. Texas became the 28th state on December 29, 1845.