The United States of America (USA), also known as the United States (U.S.) or America, is a country located primarily in North America. It is a federal republic of 50 states and the federal capital district of Washington, D.C. The 48 contiguous states border Canada to the north and Mexico to the south, with the semi-exclavic state of Alaska in the northwest and the archipelagic state of Hawaii in Oceania. Indian country includes 574 federally recognized tribes and 326 Indian reservations with tribal sovereignty rights. The U.S. asserts sovereignty over five major island territories and various uninhabited islands in the Pacific Ocean and the Caribbean. It is an ecologically megadiverse country, with the world's third-largest land area and third-largest population, exceeding 340 million.
Paleo-Indians migrated to North America across the Bering land bridge more than 12,000 years ago, and formed various cultures. Spanish Florida, the first European colony in what is now the continental U.S., was established in 1513, and later British colonization led to the first settlement of the Thirteen Colonies in Virginia in 1607. Intensive agriculture in the rapidly expanding Southern Colonies encouraged the enslavement of Africans. Clashes with the British Crown over taxation and political representation sparked the American Revolution, with the Second Continental Congress formally declaring independence on July 4, 1776. The U.S. emerged victorious from the American Revolutionary War of 1775 to 1783 and expanded westward across North America, dispossessing Native Americans during the Indian Wars. The Louisiana Purchase in 1803 and the end of the Mexican–American War in 1848 saw significant territorial acquisition. As more states were admitted, a North–South division over slavery led to the secession of the Confederate States of America, which fought the Union in the American Civil War of 1861 to 1865. With the Union's victory, slavery was abolished nationally. In the late 19th and early 20th centuries, the U.S. established itself as a great power following the Spanish–American War and World War I. After Japan's attack on Pearl Harbor in 1941, the U.S. entered World War II; its aftermath left the U.S. and the Soviet Union as the world's superpowers. During the Cold War, both countries struggled for ideological dominance and international influence. The Soviet Union's collapse and the end of the Cold War in 1991 left the U.S. as the world's sole superpower.
The U.S. national government, as established by the Constitution in 1789, is a presidential republic and liberal democracy with a separation of powers into three branches: legislative, executive, and judicial. The U.S. Congress, the national legislature, is composed of the House of Representatives (a lower house based on population) and the Senate (an upper house based on equal representation for each state). The U.S. federal system provides substantial autonomy to the 50 states, each with its own constitution and laws. The American political tradition is rooted in Enlightenment ideals of liberty, equality, individual rights, and the rule of law. Since the 1850s, the Democratic and Republican parties have dominated American politics.
A developed country, the U.S. ranks high in economic competitiveness, productivity, innovation, and higher education. The U.S. accounted for over a quarter of nominal global economic output in 2024, and its economy has been the world's largest by nominal GDP since about 1890. It possesses the largest amount of wealth of any country and has the highest disposable household income per capita among OECD countries, though U.S. wealth inequality is one of the most pronounced in those countries. A melting pot of many ethnicities and customs, the culture of the U.S. has been shaped by centuries of immigration, and its soft power influence has a global reach. The U.S. is a member of multiple international organizations and plays a leading role in global political, cultural, economic, and military affairs.
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