<P> In 1845, the Republic of Texas was annexed to the United States of America, becoming the 28th U.S. state . Border disputes between the new state and Mexico, which had never recognized Texas independence and still considered the area a renegade Mexican state, led to the Mexican--American War (1846--1848). When the war concluded, Mexico relinquished its claim on Texas, as well as other regions in what is now the southwestern United States . Texas' annexation as a state that tolerated slavery had caused tension in the United States among slave states and those that did not allow slavery . The tension was partially defused with the Compromise of 1850, in which Texas ceded some of its territory to the federal government to become non-slave - owning areas but gained El Paso . </P> <P> The Republic of Texas had formed in 1836, after breaking away from Mexico in the Texas Revolution . The following year, an ambassador from Texas approached the United States about the possibility of becoming an American state . Fearing a war with Mexico, which did not recognize Texas independence, the United States declined the offer . In 1844, James K. Polk was elected the United States president after promising to annex Texas . Before he assumed office, the outgoing president, John Tyler, entered negotiations with Texas . On February 26, 1845, six days before Polk took office, the U.S. Congress approved the annexation . The Texas legislature approved annexation in July 1845 and constructed a state constitution . In October, Texas residents approved the annexation and the new constitution, and Texas was officially inducted into the United States on December 29, 1845 . </P> <P> When Texas was annexed, Mexico broke diplomatic relations with the United States . The annexation bill did not specifically define the boundaries of Texas . The former republic claimed the Rio Grande as its southern border, while Mexican authorities had always considered the Nueces River, situated further north, to be the boundary of Mexican Texas . The United States sent John Slidell to negotiate with the Mexican government, offering $25 million ($707,115,385 today) to set the Texas border at the Rio Grande and to purchase Mexico's provinces of Alta California and Santa Fe de Nuevo México . Popular sentiment in Mexico was against any sale, and the army deposed President José Joaquín de Herrera when he appeared inclined to negotiate with Slidell . </P> <P> The United States positioned troops along the Rio Grande . On April 25, 1846, in an event known as the Thornton Affair, a large contingent of Mexican cavalry attacked an American patrol in the area between the Rio Grande and the Nueces, killing 16 Americans . On May 3, Mexican troops initiated the siege of Fort Texas, bombarding a makeshift American fort along the Rio Grande . On May 8, Zachary Taylor led 2,500 U.S. troops to relieve the fort . He was intercepted by Mexican troops, leading to the Battle of Palo Alto . Mexican troops retreated a short distance to regroup, and the following day the two sides fought fiercely in the Battle of Resaca de la Palma . The U.S. cavalry captured the Mexican artillery, and the Mexican soldiers retreated . </P>

River that mexico claimed was the texas border