Americans of English descent and Americans of Scots-Irish descent began moving into northern Florida from the backwoods of Georgia and South Carolina. Though technically not allowed by the Spanish authorities and the Floridan government, they were never able to effectively police the border region and the backwoods settlers from the United States would continue to immigrate into Florida unchecked. These migrants, mixing with the already present British settlers who had remained in Florida since the British period, would be the progenitors of the population known as Florida Crackers.
These American settlers established a permanent foothold in the area and ignored Spanish authorities. The British settlers who had remained also resented Spanish rule, leading to a rebellion in 1810 and the establishment for ninety days of the so-called Free and Independent Republic of West Florida on September 23. After meetings beginning in June, rebels overcame the garrison at Baton Rouge, now in Louisiana, and unfurled the flag of the new republic: a single white star on a blue field. This flag would later become known as the “Bonnie Blue Flag“.
In 1810, parts of West Florida were annexed by proclamation of President James Madison, who claimed the region as part of the Louisiana Purchase. These parts were incorporated into the newly formed Territory of Orleans. The U.S. annexed the Mobile District of West Florida to the Mississippi Territory in 1812. Spain continued to dispute the area, though the United States gradually increased the area it occupied. In 1812, a group of settlers from Georgia, with de facto support from the U.S. federal government, attempted to overthrow the Floridan government in the province of East Florida. The settlers hoped to convince Floridans to join their cause and proclaim independence from Spain, but the settlers lost their tenuous support from the federal government and abandoned their cause by 1813.
Seminoles based in East Florida began raiding Georgia settlements and offering havens for runaway slaves. The United States Army led increasingly frequent incursions into Spanish territory, including the 1817–1818 campaign against the Seminole Indians by Andrew Jackson that became known as the First Seminole War. The United States now effectively controlled East Florida. Control was necessary according to Secretary of State John Quincy Adams because Florida had become “a derelict open to the occupancy of every enemy, civilized or savage, of the United States, and serving no other earthly purpose than as a post of annoyance to them.”
Florida had become a burden to Spain, which could not afford to send settlers or garrisons. Madrid therefore decided to cede the territory to the United States through the Adams–Onís Treaty, which took effect in 1821. President James Monroe was authorized on March 3, 1821 to take possession of East Florida and West Florida for the United States and provide for initial governance. Andrew Jackson, on behalf of the U.S. federal government, served as a military commissioner with the powers of governor of the newly acquired territory for a brief period. On March 30, 1822, the U.S. Congress merged East Florida and part of West Florida into the Florida Territory.