On this day in 1821, the Adams-Onis Treaty was signed between the US and Spain. Spain sold Florida and access to the Mississippi River for $5,000,000 while the US gave up some of the Missouri Territory, assured Spain that Texas was theirs, and fixed the boundary with the Viceroyalty of New Spain in a way that would define Texas forever. Less than a year later, Spain recognized Mexico's independence but the Adams-Onis border conveyed to Mexico and later, the Republic of Texas. It would remain intact until the 1845 annexation of Texas and the 1848 Mexican Cession. The Gadsden Purchase in 1854 finally settled the US-Mexican border.
