While the Treaty of San Lorenzo was signed in 1795, the Spanish didn't relinquish control of this land above the 31st parallel and the Mississippi until March 1798. Thus, all of the land that Spain had used as a buffer between its own territory and the U.S. was open for grabs.
Less than a month later, on April 7, 1798, Congress rushed to fill the vacuum by creating the Mississippi Territory. Americans stepped up expansion into these lands along the Mississippi River and then along the Natchez Trace. By 1809 there were an estimated 5,000 intruders living on Chickasaw land. The territory expanded twice over the next two decades. From 1798 through 1820, the population soared from less than 9,000 to more than 222,000.