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The Adams-Onís Treaty between the United States and Spain was negotiated by Secretary of State John Quincy Adams and the Spanish Minister to the United States, Don Luis de Onís, and signed in February 1819. The principal elements in the treaty were the acquisition of Florida by the United States and the establishment of a boundary line between Spanish territory and the United States.

After the Louisiana Purchase in 1803, Pres. Thomas Jefferson argued that Florida was included in Louisiana. In 1810 and 1812 the United States annexed sections of West Florida. When John Quincy Adams became secretary of state in 1817, he sought additional territory. In 1817 and 1818 Adams and President James Monroe resumed efforts to acquire Florida and a western boundary for the Louisiana Purchase. After months of negotiations, the Adams-Onís Treaty was signed on February 22, 1819. In the treaty Spain gave East and West Florida to the United States, and the United States agreed to assume claims by citizens of the United States against Spain. In Article III the treaty stated that: "The boundary line between the two countries . . . shall begin on the Gulph [sic] of Mexico, at the mouth 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 Natchitoches, or Red River; then following the course of Rio Roxo westward, to the degree of longitude 100 west . . . , 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." In addition, the treaty stated that "all islands in the Sabine, . . . Red and Arkansas Rivers . . . [were] to belong to the United States."

By the terms of this boundary the United States agreed that Texas was on the Spanish side of the line, and Spain agreed to give up its claim to the Northwest Territory north of forty-two degrees. The treaty was approved by the U.S. Senate on February 24, 1819. However, Spanish authorities delayed their approval until 1821. The Senate approved the treaty a second time, and President Monroe ratified and exchanged it with Spanish authorities in February 1821.

The boundary line of the Adams-Onís treaty determined the southern boundary (Red River) and most of the western boundary (100th Meridian) of the future state of Oklahoma. Part of this boundary became the subject of a dispute between the United States and the State of Texas regarding claims to the area known as Greer County, enclosed between the two branches of the Red River and the 100th Meridian. In 1896 the Supreme Court of the United States ruled against Texas and stated that the disputed area was "subject to the exclusive jurisdiction of the United States of America." The area eventually became Greer, Jackson, and Harmon counties and southwestern Beckham County, Oklahoma.

Ralph Blodgett


Samuel Flagg Bemis, John Quincy Adams and the Foundations of American Foreign Policy (New York: Alfred A. Knopf, 1949).

Philip Coolidge Brooks, Diplomacy and the Borderlands: The Adams-Onís Treaty of 1819 (New York: Octagon Books, 1970).

James E. Lewis, Jr., John Quincy Adams, Policymaker for the Union (Wilmington, Del.: Scholarly Resources Inc., 2001).

Treaty of Friendship, Cession of the Floridas, and Boundaries, Treaties, Conventions, International Acts, Protocols and Agreements Between the United States of America and Other Powers, 1776–1909, Vol. 2 (Washington, D.C.: GPO, 1910).

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The following (as per The Chicago Manual of Style, 17th edition) is the preferred citation for articles:
Ralph Blodgett, “Adams-Onís Treaty,” The Encyclopedia of Oklahoma History and Culture, https://www.okhistory.org/publications/enc/entry?entry=AD004.

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