The Mississippi Territory was organized on April 7, 1798, from territory ceded by Georgia and South Carolina; it was later twice expanded to include disputed territory claimed by both the U.S. and Spain. Land was purchased (generally through unequal treaties) from Chickasaw and Choctaw Native American tribes from 1801 to about 1830.

Upon the information sent to the government by Commissioner Ellicott (see Ellicott and Gayoso), President John Adams addressed a special message to Congress, June 12, 1797, and made the following statements and recommendation: “I have determined to leave to the discretion of the officers of His Catholic Majesty, when they withdraw his troops from the forts within the territory of the United States, either to leave the works standing or to demolish them; and … I shall cause an assurance to be published and to be particularly communicated to the minister of His Catholic Majesty and to the governor of Louisiana that the settlers or occupants of the lands in question shall not be disturbed in their possessions by the troops of the United States, but on the” contrary, that they shall be protected in all their lawful claims; and to prevent or remove every doubt on this point it merits the consideration of Congress whether it will not be expedient immediately to pass a law giving positive assurances to those inhabitants who, by fair and regular grants or by occupancy, have obtained legal titles or equitable claims to lands in that country prior to the final ratification of the treaty between the United States and Spain on the 25th of April, 1796. “This country is rendered peculiarly valuable by its inhabitants, who are represented to amount to nearly 4,000, generally well affected and much attached to the United States, and zealous for the establishment of a government under their authority. I therefore recommend to your consideration the expediency of erecting a government in the district of Natchez similar to that established for the territory northwest of the river Ohio, but with certain modifications relative to titles and claims to lands, whether of individuals or companies, or to claims of jurisdiction of any individual State.”

Mississippi was the 20th state admitted to the Union, on December 10, 1817.