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? (Choice A)
A
Mexico claimed the southern border of Texas to be at the Nueces River, but the United States claimed that Texas’s border was further south, at the Rio Grande River.

(Choice B)
B
Antonio Lopez de Santa Anna sent a threatening letter to President Polk; now known as the Santa Anna Telegram, the letter so insulted Polk that he committed troops to the Mexico-Texas border.

(Choice C)
C
The people of the Texas territory desired to join the United States as a slave state, but were denied; in retaliation, Texas declared itself part of Mexico, and the United States declared war to reclaim it.