Adams-Onís Treaty

This 1819 agreement, labored over for four years by the United States and Spain, defused serious tensions on several geographic fronts between the expanding new North American country and its European counterpart, whose imperial power was fading. Spain ceded Florida to the U.S., partly to keep it out of French hands, and the U.S. recognized Spanish claims to the land comprising its Texas province and west to California and the Pacific Ocean.
More significant to Oklahoma history, Adams-Onís codified the Sabine River as the eastern boundary of Texas with America’s new Louisiana Territory, and Red River as the northern one with Indian Territory drawn from Louisiana. These mandates cleared the way for Americans, whether explorers, military expeditions, scientists, or otherwise, to travel, explore, and even settle in these environs without any threat from Spanish soldiers. They also initiated the official designation in 1820 of “Indian Territory,“ a large reserve to relocate the Native tribes from back east that Americans grew increasingly determined to have out of their way.
Adams-Onís, deriving its name from U.S. Secretary of State and future President John Quincy Adams and the Spanish Minister to the United States Don Luis de Onís, achieved another important American objective. It gave the U.S. ownership of the Sabine, Red, and Arkansas Rivers in their entirety where they separated American- and Spanish claimed territory, rather than splitting the rivers as treated boundaries normally did. American eagerness for this stemmed from earlier problems with Spain concerning the previously split Mississippi River. American ownership of Red River would figure in future disputes over oil between not-always-friendly Red River neighbors Oklahoma and Texas.
For the southeastern Indians, Adams-Onís held a different set of portents. Not least, it meant the United States could now proceed unfettered by interference from European powers toward removing the tribes from the westward tide of American settlement.

Stephen Long’s map of the Great Plains, clearly including his famed "Great American Desert,” otherwise known as present-day western Oklahoma. (Courtesy Thomas Gilcrease Institute of American History and Art)

Read the entire Oklahoma story in John J. Dwyer's The Oklahomans: The Story of Oklahoma and Its People volume 1 of a 2-part series on the 46th state and the people who make this state very special.