The dispute which started the Mexican war?

Answer 1

Several answers are available:

1. Treaty or treaties of Velasco.
2. A treaty based on a river border that few had seen or even seen mapped.
3. A small US Army Cavalry Patrol bumping into a major Mexican encampment in disputed territory.
4. A shot being fired and a war beginning ,

Answer 2

The dispute that started the Mexican-American war was Mexico's argument of where the agreed boundaries were between Texas and Mexico. Before the war, American settlers in Texas were involved in war between Mexico for Texas' independence. The most famous battle during this time was the Alamo. Texas lost this battle, but because of this hard loss, it spirited them on to win the war in the late 1830s. At this point, Texas was it's own Republic and the boundaries for this Republic lied west of the Rio Grande river.

Later, in 1845, Texas was then annexed into the United States which caused in uproar in Mexico. Mexico denied the Rio Grande as the boundaries and tried to say that the land East and South of the Nueces river was theirs, and not the United States'. This sparked the Mexican-American war in which Mexico ended up losing and giving back the land up to the Rio Grande river along with California, Nevada, New Mexico, Arizona, Utah and part of Colorado and Wyoming.