Negotiations between the two powers continued, resulting in the Treaty of 1818, which defined the Canada–United States border west of the Great Lakes. The boundary was set at the 49th parallel to the Rocky Mountains, while the territory to the west of the mountains, known as Oregon Country, would be jointly occupied. The agreement marked a watershed moment in United Kingdom–United States relations, as the United States focused on its southern and western borders and British concerns about American expansionism subsided.[74]

Adams–Onís Treaty
When Adams took office, Spanish possessions bordered the United States to the south and west. To the south, Spain retained control of Florida, which the U.S. had long sought to purchase. Spain struggled to control the Indian tribes active in Florida, and some of those tribes raided United States territory. To the west, New Spain bordered the territory acquired by the United States in the Louisiana Purchase, but no clear boundary had been established between United States and Spanish territory.[64] After taking office, Adams began negotiations with Luis de Onís, the Spanish minister to the United States, for the purchase of Florida and the settlement of a border between the United States and New Spain. 