As Secretary of State, Adams helped negotiate the Treaty of Ghent, which ended the War of 1812. He also worked with the British to establish the border between British Canada and the United States, negotiated the annexation of Florida from Spain, and composed the Monroe Doctrine, which asserted the Western Hemisphere as the US sphere of influence and warned European imperial powers not to meddle in it. ... As president, Adams supported a program to modernize the US economy. Known as the American System, it included funding for infrastructure development to facilitate trade, a tariff to protect the domestic manufacturing industry, support for a national bank and currency, and a sharp reduction in the national debt, from $16 to $5 million.

John Quincy Adams’s popularity declined as a result of his lenient approach toward Native Americans, whom he supported against the demands of westward settlers. Adams’s successor, Andrew Jackson, would go on to implement a policy of Indian removal, which involved relocating eastern tribes to lands west of the Mississippi River.

In 1828, Adams signed off on a protective tariff that became known as the Tariff of Abominations to its southern opponents, who argued that it benefited northern manufacturing interests at their expense. This led to a further decline in Adams’s popularity and opened the way for Andrew Jackson to portray Adams as an eastern establishment elite who didn't care about the interests of the frontier settler or the common man. ... In the House, Adams became one of the most vocal opponents of slavery. He consistently advocated abolitionist views and policies while condemning slavery as an immoral institution and attacking the interests of Southern slaveholders. During the Mexican-American War of 1848, Adams was one of the leading opponents of annexing Texas, presciently predicting that it would lead to civil war. In 1841, Adams appeared before the Supreme Court to argue on behalf of African slaves who had revolted and seized the Spanish ship Amistad. The Supreme Court ruling was favorable; the slaves were declared free men. The incident was a major victory for the US abolitionist movement.