What was the boundary dispute between Mexico and Texas?

The Mexican government was understandably angry over Texas's independence and its annexation to the U.S. While the Mexican government was unable to prevent Texas from joining the U.S., it continued to insist that the boundary between Mexico and Texas was the Nueces River. Texans, however, insisted that the boundary was marked by the Rio Grande, approximately 150 miles south of the Nueces.

President Polk sent diplomat John Slidell to purchase Texas from Mexico, hoping this would end Mexico's opposition to U.S. control over this land. When Mexico refused, Polk decided to settle the issue by force. Polk probably intended Slidell's mission to fail, so that the U.S. could go to war against Mexico. In the spring of 1846, Polk ordered American soldiers to enter the disputed land between the Nueces River and the Rio Grande. When Mexican troops attacked and killed several American soldiers, Polk had what he wanted: a reason to go to war against Mexico. Congress declared war on Mexico in May 1846.