When did Congress pass the Mobile Act which recognized West Florida as being part of the United States ?
The maps and journals of the explorers helped to define the boundaries during the negotiations leading to the Adams – Onís Treaty , which set the western boundary as follows : north up the Sabine River from the Gulf of Mexico to its intersection with the 32nd parallel , due north to the Red River , up the Red River to the 100th meridian , north to the Arkansas River , up the Arkansas River to its headwaters , due north to the 42nd parallel and due west to its previous boundary .