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After the French and Indian War, Britain organized East and West Florida, set West Florida’s northern line at the 31st parallel in 1763, and pushed it to 32 degrees 28 minutes a year later to take in the Natchez and Tombigbee districts. The 1783 Treaty of Paris returned the Floridas to Spain without defining those borders, so the United States claimed the 31st parallel while Spain claimed the higher line. In 1784 Spain closed New Orleans to American goods, a blow to western farmers who could not profitably haul crops over the Appalachians and depended on floating them down the Ohio and Mississippi. On October 27, 1795, in Madrid, Thomas Pinckney and Manuel de Godoy signed the Treaty of San Lorenzo.

Spain accepted the 31st parallel, made the middle of the Mississippi the western U.S. boundary, granted free navigation and the right of deposit at New Orleans, and agreed to stop arming the Chickasaw and Choctaw. Madrid gave ground because it was tied down in the French Revolutionary Wars and feared an American alliance with Britain. Andrew Ellicott surveyed the new line, and Congress created the Mississippi Territory with Natchez as its capital in April 1798. The vague Louisiana transfers that followed set off a West Florida dispute, a short-lived republic, the seizure of Mobile, and finally the 1819 Adams-Onis Treaty ceding both Floridas.

• The right of deposit let American merchants warehouse goods in New Orleans without paying customs duties while waiting for ocean ships.
• The Ellicott Stone marking the survey still stands in northern Mobile County, Alabama.
• In 1810 settlers declared the Republic of West Florida, raised a lone star flag at St. Francisville, and were annexed after about two months.
• Louisiana’s Florida Parishes still carry the name of that brief republic.
• The 31st parallel still forms state lines between Florida and Georgia, Florida and Alabama, and Louisiana and Mississippi.