What Was the Adams-Onis Treaty?

Florida Came Into the United States After Negotiations of John Quincy Adams

Engraved portrait of John Quincy Adams
John Quincy Adams. Hulton Archive/Getty Images

The Adams-Onis Treaty was an agreement between the United States and Spain signed in 1819 which established the southern border of the Louisiana Purchase. As part of the agreement, the United States obtained the territory of Florida.

The treaty was negotiated in Washington, D.C. by the American secretary of state, John Quincy Adams, and the Spanish ambassador to the United States, Luis de Onis.

Background of the Adams-Onis Treaty

Following the acquisition of the Louisiana Purchase during the administration of Thomas Jefferson, the United States faced a problem, as it was not entirely clear where the border lay between the territory obtained from France and the territory of Spain to the south.

Over the first decades of the 19th century, Americans venturing southward, including Army officer (and possible spy) Zebulon Pike, were apprehended by Spanish authorities and sent back to the United States. A clear border needed to be defined.

And in the years following the Louisiana Purchase, the successors to Thomas Jefferson, James Madison and James Monroe, sought to acquire the two Spanish provinces of East Florida and West Florida.

Spain was barely holding on to the Floridas, and was therefore receptive to negotiating a treaty which would trade away that land in return for clarifying who owned land to the west, in what today is Texas and the southwestern United States.

Complicated Territory

The problem Spain faced in Florida was that it claimed the territory, and had a few outposts on it, but it wasn't settled and it wasn't being governed in any sense of the word. American settlers were encroaching on its borders, and conflicts kept arising.

Escaped slaves were also crossing into Spanish territory, and at time U.S. troops ventured into Spain's land on the pretext of hunting fugitive slaves. Creating further complications, Indians living in Spanish territory would venture into American territory and raid settlements, at times killing the residents.

The constant problems along the border seemed likely to erupt at some point into open conflict.

In 1818 Andrew Jackson, the hero of the Battle of New Orleans three years earlier, led a military expedition into Florida. His actions were highly controversial in Washington, as government officials felt he had gone far beyond his orders, especially when he executed two British subjects he considered spies.

Negotiation of the Treaty

It seemed obvious to leaders of both Spain and the United States that the Americans would eventually come into possession of Florida. So the Spanish ambassador in Washington, Luis de Onis, had been granted full power by his government to make the best deal he could. He met with John Quincy Adams, secretary of state to President Monroe.

The negotiations had been disrupted and nearly ended when the 1818 military expedition led by Andrew Jackson ventured into Florida. But the problems caused by Andrew Jackson may have been useful to the American cause.

Jackson's ambition and his aggressive behavior no doubt underscored that Americans could be coming into the territory held by Spain sooner or later. The American troops under Jackson had been able to walk into Spainish territory at will.

And Spain, beset by other problems, did not want to station troops in remote parts of Florida to defend against any future American encroachments.

It seemed apparent that if American soldiers were to march into Florida and just seize it, there was little Spain could do. So Onis no thought thought he might as well dispense with the Florida problem while dealing with the issue of borders along the western edge of the Louisiana territory.

The negotiations were resumed and proved fruitful. And Adams and Onis signed their agreement on February 22, 1819. A compromise boundary was established between the U.S. and Spanish territory, and the United States gave up claims to Texas in exchange for Spain giving up any claim to territory in the Pacific Northwest.

The treaty, after ratification by both governments, became effective on February 22, 1821.


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McNamara, Robert. "What Was the Adams-Onis Treaty?" ThoughtCo, Dec. 2, 2016, thoughtco.com/definition-of-adams-onis-treaty-1773309. McNamara, Robert. (2016, December 2). What Was the Adams-Onis Treaty? Retrieved from https://www.thoughtco.com/definition-of-adams-onis-treaty-1773309 McNamara, Robert. "What Was the Adams-Onis Treaty?" ThoughtCo. https://www.thoughtco.com/definition-of-adams-onis-treaty-1773309 (accessed May 22, 2018).