Several signers of the declaration are imprisoned at the fort. The Treaty of Paris restores Florida to Spain. Many citizens return to St. Augustine from Havana and Mexico City. With the Louisiana Purchase, the United States believes it has a claim to west Florida. Seven years later it occupies the area.
Migration begins from the North as settlers come seeking land and opportunity.
St. Augustine is besieged by epidemics of yellow fever. The Seminole Indian Wars make settlers living on farms and plantations in outlying areas flock to the town for protection. The following year, Union troops enter St. Augustine and occupy the town for the duration of the war. St. Augustine enjoys an improved economy as Northerners looking to escape the winter's cold head south for an extended vacation season. Henry Flagler, a multi-millionaire, begins to transform St. Augustine into the American Riviera by building resort hotels and the Florida East Coast Railway.