An interesting thing about that is that the document Santa Anna signed placed the border between Texas and Mexico at the Rio Grande rather than the Rio Nueces, which had been the boundary between the two parts of Coahuila y Tejas state. Although Santa Anna was the head of state at the time (one of many between 1930 and 1850), that treaty was never "officially" accepted by Mexico. When Texas joined the US in 1845 (following another incursion into Texas by Santa Anna, briefly back as president, in 1842), this disagreement on the actual border location was one of the precipitating events of the US-Mexico War.