He was, in fact, sent down to Fort Scott because of Seminole retaliatory strikes against the United States in late 1817. General Jackson concluded that it was necessary for the United States to invade Spanish territory and forcibly take control of Pensacola, thereby dispersing (or killing) the Indians gathered there.
The Adams-Onís Treaty of 1819, also known as the Transcontinental Treaty, the Florida Purchase Treaty, or the Florida Treaty, was a treaty between the United States and Spain in 1819 that ceded Florida to the U.S. and defined the boundary between the U.S. and New Spain.