U.S. Secretary of State (1817–1825)

In pursuit of national unity, President Monroe decided a Northerner would be optimal for the position of Secretary of State, and he chose the respected and experienced Adams for the role.^[42] Adams served as Secretary of State throughout Monroe's eight-year presidency, from 1817 to 1825. Taking office in the aftermath of the War of 1812, Adams thought that the country had been fortunate in avoiding territorial losses, and he prioritized avoiding another war with a European power, particularly Britain.^[43] One of the major challenges confronting Adams was how to respond to the power vacuum in Latin America arising from Spain's weakness following the Peninsular War.^[44] Monroe and Adams agreed on most of the major foreign policy issues: both favored neutrality towards the Latin American wars of independence, peace with Great Britain, denial of a trade agreement with the French, and expansion, peacefully if possible, into the North American territories of the Spanish Empire.^[45]
Adams had begun negotiations with Britain during his time as ambassador over several contentious issues that had not been solved by the War of 1812 or the Treaty of Ghent. In 1817, the two countries agreed to the Rush–Bagot Treaty, which limited naval armaments on the Great Lakes. 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 (Oregon Country) would be jointly occupied. The agreement represented a turning point in United Kingdom–United States relations, as the U.S. turned its attention to its southern and western borders and British fears over American expansionism waned.^[46]

In the Adams–Onís Treaty, the United States acquired Florida and set the western border of the 1803 Louisiana Purchase

When Adams took office, Spanish possessions bordered the United States to the South and West. In the South, Spain retained control of Florida, which the U.S. had long sought to purchase. Spain struggled to control the Native American tribes active in Florida, some of which raided U.S. territory. In the West, New Spain bordered the territory purchased by the U.S. in the Louisiana Purchase, but no clear boundary had been established between U.S. and Spanish territory.^[43] 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 U.S. and New Spain. Adams hoped to set the U.S. border at the Rio Grande River, while the Spanish preferred that the border be set at the Mississippi River. The negotiations were interrupted by an escalation of the Seminole War, and in December 1818 Monroe ordered General Andrew Jackson to enter Florida and retaliate against Seminoles that had raided Georgia. Exceeding his orders, Jackson defeated the Seminoles, but also captured the Spanish outposts of St. Marks and Pensacola and executed two Englishmen. While the rest of the cabinet was outraged by Jackson's actions, Adams defended them as necessary to self-defense and protected by international law, and Adams eventually convinced Monroe and most of the cabinet to support Jackson. After extended negotiations, the two powers agreed to the Adams–Onís Treaty, in which Spain transferred Florida to the U.S. and the Sabine River was established as the western border of the United States. The treaty was ratified in February 1821.^[47]
As the Spanish Empire continued to fracture during Monroe's second term, Adams and Monroe became increasingly concerned that the "Holy Alliance" (which consisted of Prussia, Austria, and Russia) would seek to bring Spain's erstwhile colonies under control. In 1822, following the conclusion of the Adams–Onís Treaty, the Monroe administration recognized the independence of several Latin American countries, including Argentina and Mexico. In 1823, British Foreign Secretary George Canning suggested that the U.S. and Britain should work together to preserve the independence of these fledgling republics. The cabinet debated whether or not to accept the offer, but Adams opposed it. Instead, Adams urged Monroe to publicly declare U.S. opposition to any European attempt to colonize or re-take control of territory in the Americas, while also committing the U.S. to neutrality with respect to European affairs. Adams wrote a draft for Monroe that stated these principles and also proclaimed U.S. support for republican principles. In his December 1823 annual message to Congress, Monroe laid out the Monroe Doctrine, which was largely built upon Adams's ideas. The doctrine became one of the foundational principles of U.S. foreign policy.^[48]