Signed in February 1819 by Spain and the United States, the Transcontinental Treaty finally settled the boundaries of the Louisiana Purchase of April 1803. The United States had bought Louisiana from France with the same undefined boundaries with which France had received it from Spain. Immediately, President Thomas Jefferson wanted to open negotiations with Spain to fix the boundaries. He argued that Louisiana encompassed not just the western Mississippi Valley, but also the Gulf Coast from the Rio Grande in the west to the Perdido River in the east. With good reason, Spain considered the purchase invalid and refused to cede so much of its territory. Desultory negotiations ended entirely late in Jefferson's presidency as Spain collapsed under foreign invasion and internal turmoil. Capitalizing on this distress, the United States unilaterally annexed West Florida, as far east as the Perdido, in 1810.
In May 1816, President James Madison and Secretary of State James Monroe prepared for renewed negotiations by setting their priorities in three areas: Florida, Texas, and the Pacific Northwest. Acquiring East Florida was most important; leaving unimpeded American claims in the Pacific Northwest—an important stopover in the China trade—came second; and securing Texas from the Sabine River to the Rio Grande was least important. They also sought millions of dollars in damages claimed by American merchants against Spain. Madison and Monroe envisioned a treaty in which the United States would assume the damage claims and abandon its pretensions to Texas in exchange for Florida and the protection of its interests in the Pacific Northwest. Their desire to sign a treaty was always balanced against their effort to avoid a new war so soon after the War of 1812. Expecting their position to improve over time, Madison and Monroe did not press Spain too hard.
These priorities continued to shape policy under President Monroe and Secretary of State John Quincy Adams after Monroe's inauguration in March 1817. Monroe and Adams expected a long period of fruitless negotiations with Spain. But a series of unexpected developments at home, in Spanish Florida, and in Europe transformed Spanish thinking in 1818. At home, public and congressional opinion clamored to support the revolutionary movements in Spain's American colonies. In Florida, General Andrew Jackson seized two Spanish forts during his war against the Seminole Indians. In Europe, the Great Powers decided against intervening on Spain's behalf against its rebellious colonies. Spanish policy-makers, like their American counterparts, had calculated that time was on their side. Prolonging the negotiations would allow them to strengthen their European alliances and quiet their New World colonies. The events of 1818, however, suggested instead that they could lose Florida without receiving anything in exchange and drive the United States into support of the rebels or even war unless they made real concessions quickly.
Within months of this reevaluation, Adams and the Spanish minister in Washington, Luis de Onís, completed a treaty on the lines that Madison and Monroe had projected nearly three years earlier. The United States received Florida. The two sides fixed a boundary that ran from the Sabine River to the Pacific Ocean. And the United States assumed $5 million in damage claims of American merchants. The Spanish king delayed ratification for two years, but the treaty officially took effect in February 1821.
Because it established the first solid American claim on the Pacific, the Transcontinental Treaty has operated, along with the Monroe Doctrine, to establish Adams's claim to greatness as secretary of state. For a quarter century after its completion, however, the treaty was often seen as most significant—and most controversial—for abandoning the weak American claim to Texas.
Brooks, Philip Coolidge. Diplomacy and the Borderlands: The Adams-Onís Treaty of 1819. Berkeley: University of California Press, 1939.
Lewis, James E., Jr. The American Union and the Problem of Neighborhood: The United States and the Collapse of the Spanish Empire, 1783–1829. Chapel Hill: University of North Carolina Press, 1998.
Weeks, William Earl. John Quincy Adams and American Global Empire. Lexington: University Press of Kentucky, 1992.
James E. Lewis Jr.