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Louis Antoine Juchereau de St. Denis (September 17, 1676 – June 11, 1744) was a French-Canadian soldier and explorer best known for his exploration and development of the Louisiana (New France) and Spanish Texas regions. He commanded a small garrison at Fort de la Boulaye on the lower Mississippi River, built in 1700, and founded Fort St Jean Baptiste de Natchitoches in northern La Louisiane, as they called the French colony.
St. Denis was born at Beauport, New France (Quebec), the eleventh of the twelve children of Nicolas Juchereau (1627-1692), Seigneur du Chesnay and Saint-Roch-des-Aulnaies; member of the Sovereign Council of New France. His paternal grandfather was the elder brother of Noël Juchereau des Chatelets. His mother, Marie Thérèse Giffard de Beauport, was the daughter of Robert Giffard de Moncel, Sieur de Moncel à Autheuil, and the 1st Seigneur of Beauport, Quebec. His brother was the grandfather of Louis Barbe Juchereau de Saint-Denys (1740-1833), 1st Marquis de Saint-Denys, ancestor of Marie-Jean-Léon, Marquis d'Hervey de Saint Denys. 
St. Denis' parents apparently were able to send him to France to further his education. In late 1699, St. Denis sailed from La Rochelle with the second expedition of Pierre Le Moyne, Sieur d'Iberville (his first cousin), arriving in Louisiana. St. Denis commanded a small garrison at Fort de La Boulaye, named for a commune in the Bourgogne region of France. The fort was constructed in 1700 on the Mississippi River about 20 kilometers below the future development of New Orleans; it was designed to protect French interests against the Spanish and English in the region. St. Denis also commanded a fort at Biloxi Bay, where the French founded another settlement. St. Denis also explored to the west of the bay and up the Mississippi River, where he journeyed to the lower Red River. These expeditions to the northern areas brought St. Denis into contact with the Karankawa and Caddo tribes and taught him invaluable wilderness skills specific to the area.
Antoine Laumet de La Mothe, Sieur de Cadillac sent St. Denis and a company of French marines from Mobile in September 1713 to travel up the Red River and establish a French outpost and fort. The intent was to protect the territorial boundaries of French-Louisiana and halt the eastward expansion of the Spaniards, whose local government was based on the Rio Grande in south Texas. The Spaniards were building Fort Los Adaes about 15 miles west of Natchitoches, near the present town of Robeline, LA. St. Denis arrived in central Louisiana at what is now Natchitoches later in 1713 and built Fort St. Jean Baptiste de Natchitoches as a trading post along the banks of the Red River, whose course later changed and the result became the Cane River. He traded with the Caddo Nation there and freely sold them guns; additionally, St. Denis developed a somewhat friendly relationship with the nearby Spaniards, despite the objections of the French governship. St. Denis and his men learned many hunting and trapping skills from the Caddo Indians, who were welcoming and friendly. 
Soon after founding Natchitoches in 1714, St. Denis traveled to the lands of the Hasinai Confederacy, a group of Caddoan language tribes, and from there to Spanish outposts on the Rio Grande. At San Juan Bautista, Coahuila, Commander Diego Ramón placed St. Denis under house arrest. He confiscated his goods while awaiting instructions from Mexico City on what to do with the foreigner charged with violating Spanish trade restrictions. In the meantime, St. Denis courted and won the promise of marriage from Ramón's step-granddaughter, Manuela Sanchez-Navarro a descendant of the conquistadors of the provinces of Nueva Vizcaya and Nuevo León, Mexico. St. Denis was ordered to Mexico City and defended himself well enough to be appointed the commissary officer of the Ramón expedition charged with founding Spanish missions in east Texas. 
St. Denis returned to San Juan Bautista and married Manuela in early 1716. In the years 1716-1717 he traveled to eastern Texas to participate in the founding of six missions and a presidio. He returned to San Juan Bautista in April 1717. The death of Louis XIV and the conclusion of the War of Spanish Succession resulted in ending French-Spanish cooperation, and St. Denis returned to La Louisiane. The French sent St. Denis to Mexico City for a second time, but he escaped before being hauled to Spain as a prisoner. St. Denis made his way to Natchitoches by February 1719. Spanish officials permitted Manuela to join him in 1721, and the couple spent their remaining years at the French outpost, Le Poste des Cadodaquious, on the Red River.
From his command at Natchitoches, St. Denis was a troublesome thorn in the side of Spanish Texas. Controversy surrounds his motives to this day. St. Denis insisted that he wanted to become a Spanish subject, and his Spanish wife was proof. Suspicious Spaniards saw him as a covert agent of France. St. Denis contributed greatly to the geographical knowledge of both imperial France and imperial Spain, as well as bringing Spanish and French settlements into closer proximity and contact. His contraband trade became a way of life on the frontier and borders of Spanish Texas and French Louisiana.
On 10 January 1743, St. Denis wrote to Jean-Frédéric Phélypeaux, comte de Maurepas, at Versailles, indicating that he could no longer perform his duties as commandant of Natchitoches. He also asked permission to retire to New Spain with his wife and children, but he was forbidden to do so. St. Denis died at Natchitoches on 11 June 1744. He was survived by his wife and five children, one of whom was married briefly to Athanase de Mézières. After the time of his death, it was rumored that his wife became the richest woman west of the Mississippi River.
As St. Denis' two sons did not father any children of their own, his daughters carried his posterity. His descendants include Jefferson J. DeBlanc (1921-2007) and Alcibiades DeBlanc (1821-1883), who founded the Knights of the White Camellia, a post-Civil War white insurgent group in Louisiana. Its goal was to suppress voting by freedmen and regain political power for white men.
- Famille de Juchereau de Saint-Denys, Juchereau du Chesnay/Duchesnay
- Fehrenbach, T. R. Lone Star A History of Texas and the Texans. Collier Books: New York, 1980. pp 41-43.
- Chipman, Donald E. Spanish Texas 1519-1821. University of Texas Press: Austin, 1997. p 105