What was the main source of conflict between the united states and mexico that led to war in 1846?

Mexico claimed the Nuances River as its northeastern border, while the U.S. claimed the Rio Grande River, and the day that both troops met at the Rio Grande and the Mexican army opened fire, on April 25, 1846, the Mexican American War began.

The majority of both parties voted in favor of the bill all senate republicans voted in favor of the bill