How the U.S. Acquired Florida: The Adams–Onís Treaty of 1819

Spain ceded Florida to the United States via the Adams–Onís Treaty (signed 22 Feb 1819, ratified 1821), aka the Transcontinental Treaty. It settled borders, debts, and expansion ambitions in one diplomatic package.

Key Terms

1. Cession of Florida: East & West Florida → U.S. Spain relinquished all claims.
2. Western Boundary: Sabine River → Red River → Arkansas River → 42nd parallel → Pacific. Defined U.S. western edge (later Oregon Country).
3. Texas Sacrifice: U.S. dropped claims to Spanish Texas (west of Sabine) — a bitter pill for expansionists.
4. U.S. Citizen Claims: America assumed $5 M in claims against Spain for Napoleonic-era shipping losses.
5. Boundary Commission: Joint survey to mark the new line.

Why Spain Agreed

• Empire collapsing: Latin American wars of independence (1810–1825).
• Florida indefensible: Seminole raids, U.S. incursions (Jackson 1818).
• Needed U.S. recognition of New Spain's successor states.

Why the U.S. Wanted It

• Eliminate foreign foothold on southeast flank.
• Secure Gulf Coast, Mississippi access.
• Open path for southern slavery expansion (controversial then).

Aftermath

Florida Territory organized 1822; statehood 1845. Treaty held until Mexican–American War (1846) redrew the western line again.

A textbook case of diplomacy swapping territory for certainty.