Article: Louisiana Purchase
Section: Boundaries

Question: When was the Anglo-American Convention?

A dispute immediately arose between Spain and the United States regarding the extent of Louisiana.
The territory's boundaries had not been defined in the 1762 Treaty of Fontainebleau that ceded it from France to Spain, nor the 1800 Third Treaty of San Ildefonso ceding it back to France, nor the 1803 Louisiana Purchase agreement ceding it to the United States.
The United States claimed Louisiana included the entire western portion of the Mississippi River drainage basin to the crest of the Rocky Mountains and land extending southeast to the Rio Grande and West Florida.
Spain insisted that Louisiana comprised no more than the western bank of the Mississippi River and the cities of New Orleans and St. Louis.
The dispute was ultimately resolved by the Adams-Onis Treaty of 1819, with the United States gaining most of what it had claimed in the west.
The relatively narrow Louisiana of New Spain had been a special province under the jurisdiction of the Captaincy General of Cuba while the vast region to the west was in 1803 still considered part of the Commandancy General of the Provincias Internas.
Louisiana had never been considered to be one of New Spain's internal provinces.
If the territory included all the tributaries of the Mississippi on its western bank, the northern reaches of the Purchase extended into the equally ill-defined British possession—Rupert's Land of British North America, now part of Canada.
The Purchase originally extended just beyond the 50th parallel.
However, the territory north of the 49th parallel (including the Milk River and Poplar River watersheds) was ceded to the UK in exchange for parts of the Red River Basin south of 49th parallel in the Anglo-American Convention of 1818.
The eastern boundary of the Louisiana purchase was the Mississippi River, from its source to the 31st parallel, although the source of the Mississippi was, at the time, unknown.
The eastern boundary below the 31st parallel was unclear; the U.S. claimed the land as far as the Perdido River, and Spain claimed the border of its Florida Colony remained the Mississippi river.
In early 1804, Congress passed the Mobile Act which recognized West Florida as being part of the United States.
The Adams–Onís Treaty with Spain (1819) resolved the issue upon ratification in 1821.
Today, the 31st parallel is the northern boundary of the western half of the Florida Panhandle, and the Perdido is the western boundary of Florida.
Because the western boundary was contested at the time of the Purchase, President Jefferson immediately began to organize three missions to explore and map the new territory.
All three started from the Mississippi River.
The Lewis and Clark Expedition (1804) traveled up the Missouri River; the Red River Expedition (1806) explored the Red River basin; the Pike Expedition (1806) also started up the Missouri, but turned south to explore the Arkansas River watershed.
The maps and journals of the explorers helped to define the boundaries during the negotiations leading to the Adams–Onís Treaty, which set the western boundary as follows: north up the Sabine River from the Gulf of Mexico to its intersection with the 32nd parallel, due north to the Red River, up the Red River to the 100th meridian, north to the Arkansas River, up the Arkansas River to its headwaters, due north to the 42nd parallel and due west to its previous boundary.