On February 22, 1819, the United States purchased present-day Florida and regions of Alabama and Mississippi for $5 million U.S. dollars from Spain. In particular, John Quincy Adams, acting as U.S. Secretary of State (later elected as President), and Louis de Onís, Spanish Minister to the U.S., negotiated the deal which was named the Adams-Onís Treaty in their names (also called Florida Treaty or Transcontinental Treaty).

Spain laid claim to much of North America dating all the way back to 1493 when the Roman Catholic Pope issued a papal bull granting Spain the right to colonize parts of North America. Then, Spanish explorer, Vasco Núñez de Balboa, became the first European to reach the Pacific Ocean in the early 1500s from the Americas and he claimed all the land adjoining the Pacific Ocean for Spanish colonization. Further, the Louisiana Purchase of 1803 had failed to specify many of the boundaries which the U.S. acquired from France, and Spain and the United States disagreed over where the boundaries between the U.S. and Spanish-America laid.

So, there was a lot of history between Spain and its claims in America. However, Spain was in need of money when John Quincy Adams met Louis de Onís to negotiate the treaty.

The two countries finally came to an agreement and on February 22, 1819 the United States purchased Florida and regions of Alabama and Mississippi from Spain for $5 millions dollars. They also settled many of their boundary disputes in the Adams-Onís Treaty. For example, the Adams-Onís Treaty gave the United States all of Florida and certain surrounding regions, while Spain retained its claim to the Southwest territories west of the Sabine River including present-day Texas, New Mexico, Colorado, Arizona, Nevada, Utah, and California.