The Adams–Onís Treaty of 1819,
Florida had become a burden to Spain, which could not afford to send settlers or , so the Spanish government decided to cede the territory to the United States in exchange for settling the boundary dispute along the Sabine River in Spanish Texas. The treaty established the boundary of U.S. territory and claims through the Rocky Mountains and west to the Pacific Ocean, in exchange for the U.S. paying residents' claims against the Spanish government up to a total of $5,000,000 and relinquishing the U.S. claims on parts of Spanish Texas west of the Sabine River and other Spanish areas, under the terms of the Louisiana Purchase.
The treaty remained in full effect for only 183 days: from February 22, 1821, to August 24, 1821, when Spanish military officials signed the Treaty of Córdoba acknowledging the independence of Mexico; Spain repudiated that treaty, but Mexico effectively took control of Spain's former colony. The Treaty of Limits between Mexico and the United States, signed in 1828 and effective in 1832, recognized the border defined by the Adams–Onís Treaty as the boundary between the two nations.
By 1819, Spain was forced to negotiate, as it was losing hold on its American empire, with its western territories primed to revolt. While fighting escaped African-American slaves, outlaws, and Native Americans in U.S.-controlled Georgia during the First Seminole War, American General Andrew Jackson had pursued them into Spanish Florida. He built Fort Scott, at the southern border of Georgia (i.e., the U.S.), and used it to destroy the Negro Fort in northwest Florida, whose existence was perceived as an intolerable disruptive risk by Georgia plantation owners.
To stop the Seminole based in East Florida from raiding Georgia settlements and offering havens for runaway slaves, the U.S. Army led increasingly frequent incursions into Spanish territory. This included the 1817–1818 campaign by Andrew Jackson that became known as the First Seminole War, after which the U.S. effectively seized control of Northeastern Florida; albeit for purposes of lawful government and administration (in the state of Georgia); but not for the outright annexation of territory for Georgia; for additional US Territory; or, for the creation of another U.S. state.
Adams said the U.S. had to take control because Florida (along the border of Georgia & Alabama Territory) had become "a derelict open to the occupancy of every enemy, civilized or savage, of the United States, and serving no other earthly purpose than as a post of annoyance to them."Alexander Deconde, A History of American Foreign Policy (1963) p. 127 Spain asked for British intervention, but London declined to assist Spain in the negotiations. Some of President Monroe's cabinet demanded Jackson's immediate dismissal for invading Florida, but Adams realized that his success had given the U.S. a favorable diplomatic position. Adams was able to negotiate very favorable terms.
The United States and the Spanish Empire disagreed over the territorial boundaries of the Louisiana Purchase of 1803. The United States maintained the claim of France that Louisiana included the Mississippi River and "all lands whose waters flow to it." To the west of New Orleans, the United States assumed the French claim to all land east and north of the Sabine River..At the time of the treaty negotiations, the exploration of the western Mississippi River Basin had only begun. A modern definition of the border initially claimed by the United States begins at the mouth of Sabine Pass on the Gulf of Mexico (), thence up the Sabine River to its source (), thence due north (a distance of about along meridian 96°12'58"W) to the southern extent of the Red River drainage basin (), thence westward along the southern extent of the Red River drainage basin to the tripoint of the Red River, Arkansas River, and Brazos River (), thence northwestward along the southern extent of the Arkansas River drainage basin to the tripoint of the Mississippi River, Colorado River, and San Luis drainage basins (), thence northward along the Continental Divide of the Americas to the tripoint of the Mississippi River, Colorado River, and Columbia River drainage basins (on Three Waters Mountain) (). At this tripoint the United States claim to the Oregon Country began (see #Oregon Country.) Spain maintained that all land west of the Calcasieu River and south of the Arkansas River belonged to Tejas and Santa Fe de Nuevo México (see #New Spain.)
The United States claimed essentially the same region on the basis of (1) the voyage of Robert Gray up the Columbia River in 1792, (2) the United States Lewis and Clark Expedition of 1804–1806, and (3) the establishment of Fort Astoria on the Columbia River in 1811. On 20 October 1818, the Anglo-American Convention of 1818 was signed setting the border between British North America and the United States east of the Continental Divide along the 49th parallel north and calling for joint Anglo-American occupancy west of the Great Divide. The Anglo-American Convention ignored the Nootka Convention of 1794 which gave Spain joint rights in the region. The Convention also ignored Russian settlements in the region. The United States referred to this region as the Oregon Country, while the United Kingdom referred to the region as the Columbia District.
Between 1788 and 1793, Spain launched several more expeditions north of Alta California. On 24 June 1789, Esteban José Martínez Fernández y Martínez de la Sierra established the Spanish colony of Santa Cruz de Nuca on the northwest coast of Vancouver Island. Asserting Spain's claim of exclusive sovereignty and navigation rights, Martínez seized several ships in Nootka Sound provoking the Nootka Crisis with the British Empire. In negotiations to resolve the crisis, Spain claimed that its Nootka Territory extended north from Alta California to the 61st parallel north and from the Continental Divide west to the 147th meridian west. On 11 January 1794, the Spanish Empire and the British Empire signed the Third Nootka Convention which called for the abandonment of all permanent settlements on Nootka Sound. Santa Cruz de Nuca was formally abandoned on 28 March 1795. The Convention also called for joint and exclusive British–Spanish exploitation of the Nootka Territory. On 19 August 1796, the Spanish Empire joined the French First Republic in war against the British Empire with the signing of the Second Treaty of San Ildefonso, thus ending Spanish and British cooperation in the Americas.
East of the Continental Divide, the Spanish Empire claimed all land south of the Arkansas River that was west of the Medina River and all land south of the Red River that was west of the Calcasieu River.At the time of the treaty negotiations, the course of the Calcasieu, Red and Arkansas Rivers were only partially known and the location of the Continental Divide was yet to be determined. A modern definition of the border initially claimed by Spain begins at the mouth of Calcasieu Pass on the Gulf of Mexico (), thence up the Calcasieu River to its source (), thence due north (along meridian 93°12'47"W) to the Red River (), thence up the Red River to the meridian (99°15′19″W) of the source of the Medina River (), thence due north along that meridian (99°15′19″W) to the Arkansas River (), thence up the Arkansas River to its source (), thence due west (a distance of about along the parallel 39°15'30"N) to the Continental Divide (), thence northward along the Continental Divide presumably all the way to the Bering Strait. The vast disputed region between the territorial claims of the United States and Spain was occupied primarily by native peoples with very few Indian Trade of either Spain or the United States. In the south, the disputed region between the Calcasieu River and the Sabine River encompassed Los Adaes, the first capital of Spanish Texas. The region between the Calcasieu and Sabine rivers become a lawless no man's land. The United States saw great potential in these western lands, and hoped to settle their borders. Spain, seeing the end of New Spain, hoped to employ its territorial claims before it would be forced to grant Mexico its independence (later in 1821). Spain hoped to regain much of its territory after the regional demands for independence subsided.
The Treaty closed the first era of United States expansion by providing for the cession of East Florida under Article 2; the abandonment of the controversy over West Florida under Article 2 (a portion of which had been seized by the United States); and the definition of a boundary with the Spanish province of Mexico, that clearly made Spanish Texas a part of Mexico, under Article 3, thus ending much of the vagueness in the boundary of the Louisiana Purchase. Spain also ceded to the U.S. its claims to the Oregon Country, under Article 3.
The U.S. did not pay Spain for Florida, but instead agreed to pay the legal claims of American citizens against Spain, to a maximum of $5 million, under Article 11.The U.S. commission established to adjudicate claims considered some 1,800 claims and agreed that they were collectively worth $5,454,545.13. Since the treaty limited the payment of claims to $5 million, the commission reduced the amount paid out proportionately by 8⅓ percent. Under Article 12, Pinckney's Treaty of 1795 between the U.S. and Spain was to remain in force. Under Article 15, Spanish goods received exclusive most-favored-nation privileges in the ports at Pensacola and St. Augustine for twelve years.
Under Article 2, the U.S. received ownership of Spanish Florida (British East Florida and West Florida 1763–1783). Under Article 3, the U.S. relinquished its own claims on parts of Texas west of the Sabine River and other Spanish areas.
The Boundary Line between the two Countries, West of the Mississippi, shall begin on the Gulf of Mexico, at the Sabine Pass of the River Sabine in the Sea (), continuing North, along the Western Bank of that River, to the 32d degree of Latitude (); thence by a Line due North to the degree of Latitude, where it strikes the Rio Roxo of Nachitoches, or Red-River (), then following the course of the Rio-Roxo Westward to the degree of Longitude, 100 West from London and 23 from Washington (), then crossing the said Red-River, and running thence by a Line due North to the River Arkansas (), thence, following the Course of the Southern bank of the Arkansas to its source in Latitude, 42. North and thence by that parallel of Latitude to the South-Sea Pacific. The whole being as laid down in Melishe's Map of the United States, published at Philadelphia, improved to the first of January 1818. But if the Source of the Arkansas River shall be found to fall North or South of Latitude 42, then the Line shall run from the said Source () due South or North, as the case may be, till it meets the said Parallel of Latitude 42 (), and thence along the said Parallel to the South Sea ().At the time the treaty was signed, the course of the Sabine River, Red River, and Arkansas River had only been partially charted. Furthermore, the rivers changed course periodically. It would take many years before the location of the border would be fully determined.
South of the 32nd parallel north, the Spanish Empire and the United States settled for the U.S. claim along the Sabine River. Between the meridians 94°2′45" and 100° west, the parties settled on the Spanish claim along the Red River. West of the 100th meridian west, the parties settled on the Spanish claim along the Arkansas River. From the source of the Arkansas River in the Rocky Mountains, the parties settled on a border due north along that meridian (106°20′37″W) to the 42nd parallel north, thence west along the 42nd parallel to the Pacific Ocean.
Spain won substantial buffer zones around its provinces of Tejas, Santa Fe de Nuevo México, and Alta California in New Spain. While the United States relinquished substantial territory east of Continental Divide, the newly defined border allowed settlement of the southwestern part of the Louisiana, the Territory of Arkansaw, and the Territory of Missouri.
Spain relinquished all claims in the Americas north of the 42nd parallel north. This was a historic retreat in its 327-year pursuit of lands in the Americas. The previous Anglo-American Convention of 1818 meant that both the United States and the British Empire could settle land north of the 42nd parallel and west of the Continental Divide. The United States now had a firm foothold on the Pacific Coast and could commence settlement of the jointly occupied Oregon Country (known as the Columbia District to the rival United Kingdom). The Russian Empire also claimed this entire region as part of Russian America.
For the United States, this Treaty (and the Treaty of 1818 with Britain agreeing to joint occupancy of the Pacific Northwest) meant that its claimed territory now extended far west from the Mississippi River to the Pacific Ocean. For Spain, it meant that it kept its colony of Texas and also kept a buffer zone between its colonies in California and New Mexico and the U.S. territories. Many historians consider the Treaty to be a great achievement for the U.S., as time validated Adams's vision that it would allow the U.S. to open trade with the Orient across the Pacific.
Informally this new border has been called the "Step Boundary," although the step-like shape of the boundary was not apparent for several decades—the source of the Arkansas, believed to be near the 42nd parallel north, was not known until John C. Frémont located it in the 1840s, hundreds of miles south of the 42nd parallel.
The treaty was honored by both sides, although inaccurate maps from the treaty meant that the boundary between Texas and Oklahoma remained unclear for most of the 19th century. The American boundary was expanded in 1848 by the Treaty of Guadalupe Hidalgo with Mexico.
With the Russo-American Treaty of 1824, the Russian Empire ceded its claims south of parallel 54°40′ north to the United States. With the Treaty of Saint Petersburg in 1825, Russia set the southern border of Alaska on the same parallel in exchange for the Russian right to trade south of that border and the British right to navigate north of that border. This set the absolute limits of the Oregon Country/Columbia District between the 42nd parallel north and the parallel 54°40′ north west of the Continental Divide.
By the mid-1830s, a controversy developed regarding the border with Texas, during which the United States demonstrated that the Sabine and Neches River rivers had been switched on maps, moving the frontier in favor of Mexico. As a consequence, the eastern boundary of Texas was not firmly established until the independence of the Republic of Texas in 1836. It was not agreed upon by the United States and Mexico until the Treaty of Guadalupe Hidalgo in 1848, which concluded the Mexican–American War. That treaty also formalized the cession by Mexico of Alta California and today's American Southwest, except for the territory of the later Gadsden Purchase of 1854.Brooks (1939) ch 6
Another dispute occurred after Texas joined the Union. The treaty stated that the boundary between the French claims on the north and the Spanish claims on the south was Rio Roxo de Natchitoches (Red River) until it reached the 100th meridian, as noted on the John Melish map of 1818. But, the 100th meridian on the Melish map was marked some east of the true 100th meridian, and the Red River forked about east of the 100th meridian. Texas claimed the land south of the North Fork, and the United States claimed the land north of the South Fork (later called the Prairie Dog Town Fork Red River). In 1860 Texas organized the area as Greer County. The matter was not settled until a in 1896 upheld federal claims to the territory, after which it was added to the Oklahoma Territory.
The treaty gave rise to a later border dispute between the states of Oregon and California, which remains unresolved. Upon statehood in 1850, California established the 42nd parallel as its constitutional de jure border as it had existed since 1819 when the territory was part of Spanish Mexico. In an 1868–1870 border survey following the admission of Oregon as a state, errors were made in demarcating and marking the Oregon-California border, creating a dispute that continues to this day.