Which river did the United States claim as the southern border of Texas?

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How the Mexican-American War changed the U.S. forever — Mexican-American War Learn how the Mexican-American War ultimately pushed the United States closer to civil war.

The United States claimed the Rio Grande as the southern border of Texas.123

This claim was a central point of contention leading to the Mexican-American War, as Mexico maintained that the border was the Nueces River, which lies to the northeast of the Rio Grande.45