The United States annexed Texas in 1845, but that didn't end conflict with Mexico over the region; what dispute over Texas was a leading cause of the Mexican-American War?

The conflict started, due to a disagreement about which river was Mexico's true northern border: the Nueces or the Rio Grande.

Explanation:

Texas was annexed by the United States in 1845 and became the 28th state.

Until 1836, Texas had been part of Mexico, but in that year a group of settlers from the United States who lived in Mexican Texas declared independence. They called their new country the Republic of Texas, which was an independent country for nine years.

Politics in the United States fractured over the issue of whether Texas should be admitted as a slave or free state. In the end, Texas was admitted to the United States a slave state.

The annexation of Texas contributed to the coming of the Mexican-American War (1846-1848). The conflict started, in part, over a disagreement about which river was Mexico's true northern border: the Nueces or the Rio Grande.