How did the border dispute between Mexico and Texas affect the Mexican-American War?

Texas and Mexico disagreed over where the border lay: Texas claimed the Rio Grande, while Mexico considered the Nueces River its boundary. After the United States annexed Texas in 1845, the United States supported the Rio Grande claim, making the disputed area a source of conflict between the two countries. Thus, the correct choice is A, that the dispute became a source of conflict once Texas became part of the United States.

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