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

texas gained its independence from Mexico in 1836. Initially, the United States declined to incorporate it into the union, largely because northern political interests were against the addition of a new slave state. The Mexican government was also encouraging border raids and warning that any attempt at annexation would lead to war.

A border dispute over whether Texas ended at the Nueces River (Mexican claim) of the Rio Grande (U. S. claim)

An attack on American troops by Mexican soldiers in the disputed area between the two rivers on April 25, 1846.

The Unites States annexation of Texas in 1845