Why did the United States and Mexico go to war in 1846?
The issue that brought the conflict to open war was which river should be the border between Texas and Mexico.
Mexico was angry over the admission of Texas as a state in the United States. As long as Texas was an independent republic there was a chance that Texas would again become part of Mexico, which had a better legal claim to Texas than the United States.
The United States wanted a war with Mexico for further territorial expansion. The vision of Manifest Destiny was for an America that stretch from the Atlantic to the Pacific Ocean. To achieve this the United States need the land between Texas and California that belonged to Mexico.
The United States claimed that the border between Mexico and Texas was the Rio Grande. Mexico claimed that the border was further north. The United States sent troops into the area along the Rio Grande that was claimed by Mexico and contained Mexican Troops. The American forces clashed with the Mexican forces sparking the war.