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From Citizendium, the Citizens' Compendium
The Adams-Onís Treaty (1818), also called the Transcontinental Treaty or the Florida Treaty, between the United States and Spain, ceded Florida to the U.S. and established the boundary between the U.S. and Mexico.
U.S. Spanish Relations to 1818
Spain had long rejected repeated U.S. efforts to purchase Florida. But by 1818, Spain was facing a troubling colonial situation in which the cession of Florida made sense. Spain had been financially and politically exhausted by the European wars of Napoleon and needed to rebuild its credibility and presence in its colonies. Revolutionaries in Central and South America were beginning to demand independence. As Spain was unwilling to make any further investment in Florida, and because it worried about U.S. designs on the territory following Andrew Jackson's invasion of 1818, the sale of Florida seemed like a good course of action.
Spain had almost no military or government presence in Florida and was unable to stop Seminole warriors who routinely crossed the border and raided U.S. villages and farms or prevent runaway slaves from crossing the border southwards. During the First Seminole War, Andrew Jackson, on his own authority invaded Spanish Florida with his own militia in 1818. While his ostensible purpose that he was pursuing Indians and runaways, he also attack Spanish forts in Florida, thus provoking an international incident between the U.S. and Spain. Secretary of War John C. Calhoun privately denounced Jackson's actions while Adams saw opportunity to gain Florida.
President Monroe's Secretary of State John Quincy Adams more than anyone else was responsible for the treaty. He was the architect of a sophisticated strategy which combined diplomatic and military means to bring Spain to terms. Adams also directed an intensive public relations campaign which maintained public and Congressional support for the administration's policy.
The treaty was signed in Washington, D.C. on February 22, 1819, by Adams and the Spanish minister to the U.S. Luis de Onís. The treaty closed the first era of United States expansion by providing for the cession of East Florida, the abandonment of the controversy over West Florida (which the U.S. had seized in 1810), and defining the boundary of the Spanish province of Mexico at the Red River. Defining the boundary of Mexico at the Sabine River clearly made Texas a part of Mexico thus ending the vagueness of the boundary of the Louisiana Purchase. Spain also gave up any claims to the Oregon Territory. With the completion of this treaty, the U.S. controlled all territory on the North American continent south of the Great Lakes and East of the Mississippi River.
The U.S. did not pay Spain for Florida but did agree to assumed responsibility for the claims of American citizens against Spain as a result of the West Florida controversy to a maximum of $5 million. Pinckney's Treaty of 1795 between the U.S. and Spain was to remain in force and Spanish goods were to receive certain tariff privileges in Florida ports.
The new boundary between the U.S. and Spanish Mexico ran along the Sabine River from the Gulf of Mexico north to the 32nd parallel; then north to the Red River, along it to the 100th meridian; north to the Arkansas River and along it to its source; then north to the 42nd parallel; and then west on that line to the Pacific Ocean.
The ministers of King Ferdinand VII at first refused to ratify the treaty. The Spanish argued that the U.S. had already broken the treaty by James Long's 1819 expedition into Texas. In order to placate Madrid, Washington disavowed Long's actions. Spain's real goal was to stop American recognition of the independence of breakaway colonies in Latin America. When Ferdinand lost some powers and became a constitutional monarch in 1820, his council was obliged to approve the treaty. Ratification became official in 1821.
In order to implement the treaty, Congress established a claims commission (1821-1824) to handle Americans' claims against Spain. Many notable lawyers, including Daniel Webster and William Wirt, represented claimants before the commission. During its time, the commission examined 1,859 claims arising from over 720 spoliation incidents and distributed the $5 million in a fair manner.
The treaty reduced tensions between the U.S. and Spain (and also, after 1821, between the U.S. and Mexico) and allowed budget cutters in Congress to reduce the army budget and reject the plans to modernize and expand the army as proposed by Secretary of War John C. Calhoun.
The treaty was ratified by Mexico in 1831 and was honored until it was replaced by the Treaty of Guadalupe-Hidalgo in 1848 after the Mexican-American War. Inaccurate maps from the treaty, however, meant that the boundary between Texas and Oklahoma remained unclear for most of the 19th century.