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He was Amazon.com's first-ever history editor and has bylines in New York, the Chicago Tribune, and other national outlets. our editorial process Robert McNamara Updated January 09, 2020 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 present-day 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. The treaty was viewed as a significant event at the time, and contemporary observers, including former president Thomas Jefferson, lauded the work of John Quincy Adams. 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, was apprehended by Spanish authorities and sent back to the United States. A clear border needed to be defined before minor incidents on the border escalated into anything more serious. 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 (the regions had been loyal to Britain during the American Revolution, but following the Treaty of Paris, they reverted to Spanish rule). 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 the region wasn't being governed in any sense of the word. American settlers were encroaching on its borders, essentially squatting on Spanish land, and conflicts kept arising. Escaped slaves were also crossing into Spanish territory, and at the 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. Andrew Jackson at the Battle of New Orleans. 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 reinforced the fear that of the Spaniards 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 Spanish territory at will. Spain was beset by other problems. And it did not want to station troops, which would have to be supplied, in remote parts of Florida to defend against any future American encroachments. There was no escaping that if American soldiers could march into Florida and just seize it, there was little Spain could do. So Onis thought he might as well dispense with the Florida problem entirely 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. The treaty was eventually followed by other treaties that essentially confirmed the boundaries set out in 1821. An immediate result of the treaty was that it reduced tensions with Spain, and made the likelihood of another war seem remote. So the military budget of the United States could be cut and the size of the U.S. Army reduced in the 1820s.