Why did the Rio Grande become a disputed border between Texas and Mexico in the 1840s?

The Rio Grande became a disputed border between Texas and Mexico in the 1840s due to conflicting territorial claims following Texas's independence from Mexico in 1836.1

While the United States asserted that the Rio Grande served as the legitimate southern boundary of Texas, Mexico maintained that the border was the Nueces River, which lies approximately 150 miles to the northeast. This disagreement persisted after the United States annexed Texas in 1845, leading to a breakdown in diplomatic relations.1

The tension escalated when U.S. President James K. Polk ordered troops into the disputed territory between the two rivers. Following a skirmish between Mexican and American forces in April 1846, the United States declared war. The Mexican-American War concluded in 1848 with the Treaty of Guadalupe Hidalgo, which formally established the Rio Grande as the international boundary.123