After the American Revolution, Spain controlled Florida again, but it struggled to police the border. Raids, disputes over runaway enslaved people, and clashes involving Seminoles and other groups increased along the Georgia–Florida frontier, setting the stage for a major crisis.
The First Seminole War is generally dated to 1817–1818. Fighting involved U.S. forces and Seminole communities in Spanish Florida, along with allied and neighboring groups, and it increased pressure on Spain’s already weak hold over the territory.
In spring 1818, General Andrew Jackson led U.S. troops into Spanish Florida, attacking Seminole villages and taking key Spanish posts. His actions created major diplomatic tensions, but they also demonstrated Spain’s difficulty defending Florida and helped push negotiations forward.
After Jackson’s 1818 occupation of Pensacola, the United States later returned the town to Spain. The episode highlighted how quickly the border conflict could turn into a broader international dispute if the Florida question was not settled.
On February 22, 1819, U.S. Secretary of State John Quincy Adams and Spanish minister Luis de Onís signed the Treaty of Amity, Settlement, and Limits (also called the Adams–Onís or Transcontinental Treaty). Spain agreed to cede Florida to the United States, while the two countries also worked to clarify their North American boundary claims.
Beyond Florida, the agreement defined a boundary line running from the Gulf region north and west, then across the continent near latitude 42°N to the Pacific. This helped settle competing U.S. and Spanish claims in North America and shaped later disputes over the Pacific Northwest.
Two days after the treaty was signed, the U.S. Senate ratified it. However, the treaty still could not take effect until Spain completed its own ratification, which did not happen right away.
Spain ratified the treaty on October 24, 1820, after delays that reflected Spain’s political instability and concerns about its other American colonies. Ratification cleared the way for the treaty to become fully binding once both sides exchanged their approved documents.
Because Spain’s ratification took so long, the United States ratified the treaty again in early 1821. This step helped remove procedural and political obstacles before the final exchange of ratifications.
On February 22, 1821, the treaty was proclaimed and entered into force, making the Florida cession official. With the treaty in effect, U.S. leaders could formally move from negotiation to taking control and setting up governance in the new territory.
President James Monroe appointed Andrew Jackson as U.S. commissioner to take possession of Florida and granted him the powers of governor during the transition. This was meant to provide quick, centralized control while a permanent territorial government was organized.
In July 1821, Spain carried out the physical transfer of authority, including formal ceremonies in key cities. On July 17, 1821, the transfer in Pensacola marked the end of Spain’s rule there and the start of U.S. control under the treaty’s terms.
On March 30, 1822, the United States organized the Florida Territory, merging East Florida with part of West Florida into a single territorial government. This step moved Florida from a military transition period into a more stable civil administration under U.S. law.
Adams–Onís Treaty and the Acquisition of Florida (1819–1821)