The Old Southwest was the early name for the territories that were acquired by the United States from Spain in the Adams-Onis Treaty (also called the Transcontinental Treaty) of 1819. The region included present–day Florida and the southern parts of Alabama and Mississippi.
The Spanish who first colonized the Old Southwest referred to the region simply as East and West Florida. Unfortunately, the Spanish monarch had only a weak hold on the Floridas and many Americans had settled there. In 1811 U.S. settlers in West Florida rebelled and declared their independence. President James Madison (1809–1817) ordered the Governor of Orleans Territory (gained in the Louisiana Purchase of 1803) to take possession of West Florida. This action provided impetus for the U.S. government to claim the East Florida territory, which would allow the country to organize all its territory east of the Mississippi River.
Negotiations between Spain and the United States were led by John Quincy Adams (1767–1848), the U.S. Secretary of State, and Luis de Onis (1762–1827), Spain's Minister to the United States. Though the United States gained the Old Southwest, it also made some concessions to Spain in establishing the boundary between Spanish and American claims from Texas to the Pacific Ocean. The treaty was signed in Washington, D.C., in 1819 and was approved by the governments of both countries two years later. Plantation owners largely settled the Old Southwest during the 1800s. Later the region became the state of Florida (admitted to the Union in 1845) and formed parts of what would become Alabama (1819) and Mississippi (1817).
"Old Southwest." Gale Encyclopedia of U.S. Economic History. . Encyclopedia.com. (September 26, 2018). http://www.encyclopedia.com/history/encyclopedias-almanacs-transcripts-and-maps/old-southwest
"Old Southwest." Gale Encyclopedia of U.S. Economic History. . Retrieved September 26, 2018 from Encyclopedia.com: http://www.encyclopedia.com/history/encyclopedias-almanacs-transcripts-and-maps/old-southwest