What was the source of the conflict between the united States and Mexico before they went to war?

The conflict between the United States and Mexico before the war primarily stemmed from territorial disputes and the annexation of Texas. After Texas won its independence from Mexico in 1836, the U.S. annexed it in 1845, which Mexico viewed as a violation of its sovereignty. Additionally, both nations had differing claims over territories in the southwest, including present-day California and New Mexico. These tensions ultimately escalated into the Mexican-American War in 1846.