Adams-Onís Treaty Acquires Florida Through Coerced Spanish Cession After Jackson's Unauthorized Invasion

Secretary of State John Quincy Adams and Spanish Minister Luis de Onís sign the Adams-Onís Treaty (also known as the Transcontinental Treaty or Florida Purchase Treaty) in Washington, D.C., under which Spain cedes Florida to the United States and establishes a boundary line extending to the Pacific Ocean. The treaty emerges from Spain’s weakened position after General Andrew Jackson’s unauthorized 1817-1818 invasion of Spanish Florida, capture of Spanish forts, and execution of two British subjects. Following tense discussions inside the Monroe administration about Jackson’s flagrant violation of Spanish sovereignty, President Monroe decides to back Jackson rather than apologize to Spain, effectively using military aggression as leverage in treaty negotiations. Onís, fearing the United States might seize Florida outright through further military action, agrees to cede the territory.

Under the treaty’s principal terms, the United States takes possession of both West and East Florida, in return for agreeing to pay legal claims of American citizens against Spain up to $5 million—meaning American taxpayers effectively purchase Florida to compensate American claimants rather than Spain receiving payment. The U.S. abandons its claims to Texas west of the Sabine River, while Spain gives up its claims to the Oregon Country, establishing a boundary that extends American territorial claims to the Pacific Ocean. The treaty represents the first international agreement legitimizing an American foothold on the Pacific, and when ratified (by the U.S. Senate in 1819 but delayed by Spanish authorities until 1821), the United States spans the width of the continent from the Atlantic to the Pacific.

The Adams-Onís Treaty demonstrates how military aggression and coercive diplomacy drive American territorial expansion under the guise of legitimate treaty-making. Florida had become a burden to Spain, which after being exhausted by the Peninsular War (1807-1814) against Napoleon could not afford to send settlers or staff garrisons, but the treaty is less a negotiated agreement than a coerced cession extracted through the threat of further American military invasions. The acquisition serves slavery expansion by eliminating Spanish Florida as refuge for escaped slaves and opening new territory for plantation development, while also removing indigenous peoples’ sanctuary and facilitating their dispossession. The treaty establishes precedents for using unauthorized military actions as diplomatic leverage and for continental expansion through coerced agreements that recur from the Mexican-American War through the Spanish-American War, revealing how territorial acquisition often depends on military force or its threat disguised as peaceful treaty negotiation.