Florida was formally annexed into the United States by the Adams-Onis Treaty of 1819 between the US and Spain. However, Florida’s move from Spanish Colony that spanned from the Mississippi River to the Keys after the 1783 Treaty of Paris to American territory happened piecemeal between 1810 and 1819 and involved more than anything the exploits of future President, General Andrew Jackson. Below is a History Channel Documentary I have on DVD but found on YouTube that tells the story far better than I can.