The war intensified the debate over slavery in the United States, contributing to bitter debates that culminated in the American Civil War (1861–1865).
In Mexico, the war came in the middle of continued domestic political turmoil, which increased into chaos during the conflict.
Mexico threatened war with the United States if it annexed the Republic of Texas. 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.
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.
The military defeat and loss of territory was a disastrous blow, causing Mexico to enter "a period of self-examination ...
as its leaders sought to identify and address the reasons that had led to such a debacle." In the immediate aftermath of the war, some prominent Mexicans wrote that the war had resulted in "the state of degradation and ruin" in Mexico, further claiming, for "the true origin of the war, it is sufficient to say that the insatiable ambition of the United States, favored by our weakness, caused it." The shift in the Mexico-U. border left many Mexican citizens separated from their national government.
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.
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.
After independence from Spain in 1821, Mexico contended with internal struggles that sometimes verged on civil war and the northern frontier was not a high priority.