A lot of shenanigans and the Adams-Onis Treaty of 1819 established the Red River as the southern boundary of the Louisiana Purchase of 1803.
Every time the "Adams-Onis Treaty" of 1819 is mentioned, it is invariably linked to Florida. The U.S. Office of the Historian even labels it the "Acquisition of Florida."
And while yes, the transfer of Florida to the United States was one of the ultimate results of the 1819 treaty between the United States and Spain, it was not the ONLY outcome, y'all. Read on about how it took almost two decades to establish the southern border of Louisiane.
Where oh Where is Louisiana?
The Louisiana Purchase of 1803 had a clearly defined eastern border: the Mississippi River. But no other borders were as definitively described; the land that the US acquired from France (and which prior to France regaining the territory, was claimed and governed by Spain) was all the earth between the northern-most and southern-most rivers that emptied into the Mississippi from the west.
This was a huge land purchase that needed defined southern, western, and northern borders. And since this website deals with the Red River of the South, the southern-most river to enter the Mississippi River from the west, we'll concentrate on how this river kinda-sorta became the border between the United States and Spain with the Adams-Onis Treaty of 1819 (also known as the Transcontinental Treaty, ratified in 1821).
Caddo, France, Spain!
In 1803, Spain claimed the province of Texas. But its reach in today's northern Texas was murky: the Caddo and Wichita homelands that stretched along the Red River in SW Arkansas, NW, Louisiana, SE Oklahoma, and NE Texas had allied themselves with France, which had built several trading posts and a few fortifications along the Red River. Now why'd they go do that for?
In 1714, Louis Juchereau de St. Denis, a Canadian Frenchman, was tasked by the French government to open a trading post with the Natchitoches (Caddoan) tribe along the Red River in today's central Louisiana to counter the Spanish mission at Los Adaes, established in 1716 less than twenty miles west of the Natichtoches village. Tensions between the French and Spanish brewed in this region until 1718, when Juchereau married Manuela Sanchez-Navarro, the stepdaughter of a Spanish commander at the Rio Grande River. This strategic alliance allowed peace and trade to prevail in the Red River region, to an extent.
By 1719, the Frenchman Bernard de la Harpe had established trading posts and forts within the Caddos homelands along the Red River above the Great Raft north of Natchitoches. These trading sites gave the French colonial dominance. Since the French traded guns for furs and hides, they were welcomed by the Caddo men, who disliked the emphasis on sacrifice, humility, and domestics that the Spanish missions like Los Adaes offered. Then, a French raid on Los Adaes in 1719 (called the "Chicken War") proved that the mission was unsafe, leading the Spanish to close down the mission and fortification. During this time of upheaval, rumors spread that the French had invaded Florida and were marching towards Texas, so Spain consolidated its Texas subjects into Bexar (San Antonio). Then, Spanish troops were defeated by the Taovayans and Comanches (los Nortenos) at the dual villages along the Red River in 1759 after an Indian raid on the Apache-focused mission at San Saba. All of these actions proved to Spain that it could not gain a solid foothold along the Red River.
By 1762, Spain's failure to control the Red River wasn't a big deal anymore, as Spain had gained Louisiane at the end of the Seven Years Wars (in the U.S., the "French-Indian War.") But memory runs deep, and to keep peace in the region, the Spanish government allowed the French-Catholic churches to control most of the administration along the Red River instead of establishing national dominance. While Spain built a few forts for defensive purposes, trade continued with the French-Indian alliances. Then, in 1800, Louisiane was transferred back to France upon the Treaty of Ildefonso in exchange for Spain gaining some European lands.
The United States enters the Chat.
The US was barely its own country by 1800, and as a former English colony, the center of its government was all the way in the mid-Atlantic seaboard and nowhere near Louisiane. But after the French-Indian War (1754-1763) and the American Revolution (1776-1783), the United States could claim all rivers that drained into the Mississippi River from the east. This meant that they could, technically, reach the Gulf of Mexico. But Spain held fast onto "Spanish Florida," which consisted of the Floridian peninsula, the southern coastal lands of today's Alabama and Mississippi, and all of today's Louisiana. This meant that the US had to seek Spain's permission to enter New Orleans via the Mississippi River. It did this with the Treaty of San Lorenzo of 1795, also called Pickney's Treaty, which allowed Americans to have access to the New Orleans port and the Mississippi River.
With the 1800 Treaty between Spain and France, which once again gave France control over Louisiane, the U.S./Spain Treaty of 1795 was void. And this is how the US was able to purchase Louisiane in 1803... it had wanted to purchase rights to New Orleans but the French offered the entire territory instead.
Americans on the Move
When it comes to colonialism, historians have a simplistic way of explaining motivations of the three main colonial empires in the New World: the Spanish sought Gold, the French sought Trade, and the English sought Land. While all three nations practiced chattel slavery and introduced Abrahamic religion to achieve their objectives, their primary greed dictated HOW they colonized.
The United States inherited the land hunger from its former colonial overlord. With the Louisiana Purchase of 1803, Americans immediately began to move westward to take, cultivate, and profit from the lands. This meant that they were coming into places like New Orleans, Natchitoches, and the Caddo homelands above the Great Raft. The Spanish were alarmed that their claims on Texas might be challenged by the United States, as the extent of the Red River's watershed hadn't been established. And, technically, the French had physically staked their claims with the trading posts and forts along the Red River in the Caddo nation, which the US could now claim, thus loosening any remnants of control may have had.
But Spain, reeling from the Napoleonic Wars in Europe, couldn't afford another war in the New World. It also considered itself somewhat of an ally of the United States in their mutual dislike of England, and the fact the US had sided with the colonial powers against Santo Domingo/Haiti during their revolution of 1795. And the Caddos had vacated much of their homelands due to raids by the Osages, so the French had abandoned the posts and forts above the Great Raft, making the American claim to the Red River also precarious. Ergo, Spain and the US came to an agreement instead.
The Adams-Onis Treaty, finally
In 1819, one hundred years after the French established a colonial dominance along the Red River Valley, Spain and United States drew boundaries around the southern portion of the Louisiana Territory to ensure their own colonial claims, the Caddos be damned. John Quincy Adams, Secretary of State during the James Monroe Presidency, and Spanish diplomat Luis De Onis y Gonzalez Vara negotiated the treaty that gave the United States Spanish Florida in exchange for a set boundary around the Caddo homelands at the Red River a promise that Americans would not make incursions into Texas.
West of the Great Bend and the Little River's watershed, the Red River acted as a southern border between the nations until the 100th Meridian, which then formed, partially, the western border until the Continental Divide. North of the Sabine River extending through the Sulphur River and to the Red River was a straight line that formed Texas's northeastern boundary; this "Index Line," as it was called, was not successfully surveyed until two decades later.
This delay in establishing boundaries led to a veritable "no man's land" between Spanish Texas and Louisiana, with some consequences of national importance: Spanish spies! Double agents! Treason! Criminals! And pirates!
A Land Smorgasbord
The Adams-Onis Treaty of 1819 didn't really stick, either. By 1821, Mexico had become independent from Spain, so the US and Mexico had to renegotiate the boundaries between the nations, which was further complicated by the fact that many Americans were coming into northeastern Texas due to lax border protections. These actions led to Texas becoming a Anglo-centric, independent country from Mexico a mere fifteen years later (for those who are doing the math: 1836). Then, just ten years after that (1845), the US and Mexico would fight over the southern extent of Texas's border, which ultimately culminated in the Mexican Cession of 1848.
The short-lived Adams-Onis Treaty of 1819 had major historical implications both before and after its demise, with the Red River playing an outsized role in this drama.
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