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Why did the United States and Mexico disagree over the Texas border?
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Mexican-American War
Learn how the Mexican-American War ultimately pushed the United States closer to civil war.
The disagreement over the Texas border stemmed from conflicting territorial claims following the Texas Revolution, during which Texas secured de facto independence from Mexico in 1836. Although Mexico refused to formally recognize Texas's sovereignty, the region sought to expand its western territory, leading to an ill-defined boundary.12
By the mid-1840s, the dispute centered on two specific rivers:
- The United States position: Following the 1845 annexation of the Republic of Texas, the U.S. asserted that the southern boundary of Texas was the Rio Grande.34
- The Mexican position: Mexico maintained that the historical boundary of the region was the Nueces River, which lies approximately 150 miles to the northeast of the Rio Grande.34
This impasse became a primary catalyst for the Mexican-American War. U.S. President James K. Polk eventually ordered troops into the disputed territory between the two rivers, and a subsequent skirmish in April 1846 provided the rationale for the U.S. to declare war.134