river that mexico claimed was the texas border
History of Texas (1845–1860) 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.