Answer:
The main, legitimate reason for the start of the Mexican-American War was because the US wanted to claim more western land for itself. However, the official reason for the Mexican-American War was under the pretext that Texas, which used to be a territory under Mexico, and who won and established the Republic of Texas, took control of "extra" land under the treaty, and that the real border farther north then the Rio Grande. With the introduction of the Republic of Texas into the United States, the problem grew proportionally from the Republic vs Mexico to the United States vs Mexico.