After the Republic of Texas was added as the 28th states, the American and Mexican governments disagreed if the border do Texas ended at the Neuces or at the Rio Grande rivers. In November, Polk sent U.S. Diplomat John Slidell to Mexico to seek boundary adjustments in return for the U.S. government’s settlement of the claims of U.S. citizens against Mexico, and to so that an offer could be made to purchase California and New Mexico. After the mission failed, the U.S. army, under General Zachary Taylor, advanced to the mouth of the Rio Grande, the river that the state of Texas claimed as its southern boundary.
Mexican Tripp’s killed nine U.S. soldiers and captured others who were in the the area that Mexico claimed as their own territory, and not in the state of Texas. President James Knox Polk advocated for war and on May 13, 1846, the U.S. Congress voted fo declare war against Mexico over the territorial dispute. During the Mexican American War, President Polk offered to buy California and New Mexico, from the Mexican government. Mexico refused to make the land sale, until the country surrender at the end of the war.