Answer:
After American settlers poured into Spanish Florida following the War of 1812, Spain decided to cede Florida to the United States.
Explanation:
In the early 19th century, Spain attempted to attract new settlers with generous lands in Florida, and colonists from both the United States and Spain came to the country in large numbers. After attacks by Europeans on Indian villages, Indians began to raid settlements in the Georgia colony - allegedly at the behest of the Spaniards. The United States Army increasingly invaded the Spanish colony, including during the campaign against the Seminoles in 1817/1818 under Andrew Jackson, who became known as the First Seminole War. After this war, the United States practically controlled East Florida.
On February 22, 1819, the Adams-Onis Treaty was signed by the United States and Spain and became effective on July 10, 1821. Under the terms of this treaty, the United States received the areas of Florida and in return waived all claims in Texas.