How did the Rio Grande border dispute contribute to the Mexican-American War?

The border dispute over the Rio Grande was a primary catalyst for the Mexican-American War. Following the 1845 annexation of the Republic of Texas by the United States, a fundamental disagreement emerged regarding the southern boundary of Texas: the United States claimed the border was the Rio Grande, while Mexico insisted it was the Nueces River, located approximately 150 miles to the northeast.123

This tension escalated when U.S. President James K. Polk sent troops under General Zachary Taylor to occupy the disputed territory between the two rivers in early 1846. After Mexico refused to negotiate the border and other territorial claims, a skirmish occurred in April 1846 when Mexican troops crossed the Rio Grande and engaged Taylor’s forces. Polk utilized this event to secure a declaration of war from Congress, famously asserting that Mexico had invaded U.S. territory and shed American blood on American soil. The conflict ultimately concluded with the Treaty of Guadalupe Hidalgo, which officially established the Rio Grande as the border.243