Why did the U.S. and Mexico go to war?

Answer 1

The immediate reason for the Mexican-American War was that the two countries could not agree on which river formed the southern boundary of Texas. The Texans, and thus the US, claimed land as far south as the Rio Grande, but the southern boundary of the Mexican province of Texas (before it won its independence from Mexico) had been along the Nueces river, somewhat further north, and that was where the Mexicans said the border was.

Mexico had also threatened war if the Republic of Texas was annex by the US, which it was in 1845.

Many people in the US, including President Polk, also wanted to incorporate other northern Mexican territory, including California, in the US