The colonies of East Florida and West Florida remained loyal to the British during the war for American independence. By the Treaty of Paris in 1783 they had returned to Spanish control. After 1783, Americans immigrants began moving into West Florida.
In 1810, American settlers in West Florida rebelled, declaring independence from Spain. President James Madison and Congress used the incident to claim the region, knowing that Napoleon’s invasion of Spain had seriously weakened the Spanish government. The United States claimed that West Florida was part of the Louisiana Purchase of 1803. Negotiations over Florida began with the mission of Don Luis de Onís to Washington in 1815. The issue was not resolved until Monroe became president and John Quincy Adams his Secretary of State.
U.S.-Spanish relations were tense due to Spain’s suspicion of American support for the independence struggles of Spanish-American colonies. The situation became critical when General Andrew Jackson seized the Spanish forts at Pensacola and St. Marks in his 1818 authorized raid against Seminoles and escaped slaves. Jackson executed two British citizens on charges of inciting the Indians and runaways. Monroe’s government considered denouncing Jackson’s actions, but Adams defended Jackson, claiming it was necessary to restrain the Indians and escaped slaves.
Adams used Jackson’s military action to present Spain with a demand either to control the inhabitants of East Florida or to surrender it to the United States. Minister Onís and Secretary Adams reached an agreement in which Spain ceded East Florida to the United States and renounced all claim to West Florida. Spain received no compensation, but the United States agreed to assume liability for five million dollars in damages by American citizens who rebelled against Spain. Under the Onís-Adams Treaty of 1819 the United States and Spain defined the western limits of the Louisiana Purchase. Spain also surrendered its claims to the Pacific Northwest. In return, the United States recognized Spanish sovereignty over Texas.
Source: Acquisition of Florida: Treaty of Adams-Onis (1819) and Transcontinental Treaty (1821)
Office of the Historian, U.S. Department of State