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 served as military governor of the newly acquired territory, but only for a brief period. On March 30, 1822, the United States merged East Florida and part of West Florida into the Florida Territory.
When did East and West Florida Merge 
On March 30, 1822, the United States merged East Florida and part of West Florida into the Florida Territory