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The United States of America (USA or U.S.A.), commonly known as the United States (US or U.S.) or America, is a country primarily located in North America, between Canada and Mexico. It is a federation of 50 states, a federal capital district (Washington, D.C.), and 326 Indian reservations that overlap with state boundaries. Outside the union of states, it asserts sovereignty over five major unincorporated island territories and various uninhabited islands. The country has the world's third-largest land area, largest maritime exclusive economic zone,
and the third-largest population, exceeding 334 million.Paleo-Indians migrated across the Bering land bridge more than 12,000 years ago. British colonization led to the first settlement of the Thirteen Colonies in Virginia in 1607. Clashes with the British Crown over taxation and political representation sparked the American Revolution and the Revolutionary War of 1775–1783. The Second Continental Congress voted for independence and formally declared independence on July 4, 1776. The country began expanding across North America. As more states were admitted, sectional division over slavery led to the secession of the Confederate States of America, which fought the remaining states of the Union during the 1861–1865 American Civil War. With the Union's victory and preservation, slavery was abolished nationally. By 1900, the United States had established itself as a great power, becoming the world's largest economy. After Japan's attack on Pearl Harbor in December 1941, the U.S. entered World War II. The aftermath of the war left the U.S. and the Soviet Union as the world's two superpowers and led to the Cold War, during which both countries engaged in a struggle for ideological dominance and international influence. Following the Soviet Union's collapse and the end of the Cold War in 1991, the U.S. emerged as the world's sole superpower.
The U.S. national government is a presidential constitutional republic and liberal democracy with three separate branches of government: legislative, executive, and judicial. It has a bicameral national legislature 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. Substantial autonomy is given to states and several territories, with a political culture that emphasizes liberty, equality under the law, individualism, and limited government.
One of the world's most developed countries, the United States ranks among the highest in the world in international measures of income, wealth, economic competitiveness, productivity, innovation, human rights, and higher education. It has the highest median income per capita of any non-microstate and possesses by far the largest amount of wealth of any country, with the American economy the largest nominally and accounting for over a quarter of global GDP. The U.S. is a founding member of the World Bank, International Monetary Fund, Organization of American States, NATO, Quadrilateral Security Dialogue, WHO, and a permanent member of the United Nations Security Council.