1819 Adam-Onis Agreement
The United States acquired Florida under the terms of the Adam Onis treaty with Spain. In return, the US government agreed to assume $5 million worth of debts for which the Spanish were liable. In addition, the US recognized the southwestern border between itself and the Spanish Empire at the Sabine River.
After Jackson's attack on the Indians in Florida, it was clear that Spain could no longer maintain any semblance of control over the area. The United States government presented the Spanish with two alternatives: either they could bring the territory under control, or the United States would seize it. The Spanish decided that they would sell the area to the United States, rather than lose the territory in a war.
The Spanish agreed to sell the territory of Florida; as well as any claims they might have on the Oregon Territory; to the United States. In return, the US agreed to pay $5 million and assume any debts in the United States for which the Spanish government might be liable.
The treaty was signed in Washington by Secretary of State John Adams and Luis de Onis the Spanish Minister of February 22, 1819. The Spanish held up ratification and thus the US Senate had to ratify it twice once immediately on it's signing the second time two years later after the Spanish had approved, on February 19, 1821