1820

1820 was a leap year in the Gregorian calendar. It occurred amid the post-Napoleonic realignment of political authority and a wave of liberal and nationalist movements across the Atlantic world.

In the United States, Congress passed the Missouri Compromise in March, admitting Missouri as a slave state and Maine as a free state, and establishing the 36°30′ parallel as the dividing line for future territorial states to maintain sectional balance. The year also featured James Monroe's reelection as president after an essentially unopposed campaign, an omen of the Era of Good Feelings. The nation conducted the decennial census of 1820, documenting a growing population.

In Europe, liberal uprisings produced constitutional change in several countries. In Spain, the liberal revolution of 1820 led to the restoration of the 1812 constitution and sparked international intervention when authorities attempted to reimpose absolutism. In Portugal, the Liberal Revolution of 1820 established a constitutional framework, influencing reforms across its empire.

In the Americas outside the United States, independence movements and post-colonial reorganizations continued to unfold, reshaping national borders and governance as former colonies moved toward constitutional rule and republican government.

Culturally and scientifically, the period continued to see gradual modernization, with expanding print and educational networks, and continued debates over human rights, representation, and the role of the state in society.