The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic consisting of 50 states and a federal capital district, Washington, D.C. The 48 contiguous states border Canada to the north and Mexico to the south, with the semi-exclave of Alaska in the northwest and the archipelago of Hawaii in the Pacific Ocean. The United States also asserts sovereignty over five major island territories and various uninhabited islands in Oceania and the Caribbean. It is a megadiverse country, with the world's fourth-largest land area and third-largest population, exceeding 341 million.
Paleo-Indians migrated from North Asia to North America around 15,000 years ago and formed various civilizations. Beginning with the 1607 settlement of Virginia, British colonization established the Thirteen Colonies. The American Enlightenment, spread throughout the colonies in the 18th century, valued republicanism and liberalism. Clashes with the British Crown over taxation without parliamentary representation, among other denied English rights, sparked the American Revolution, which led to the July 2, 1776, Lee Resolution formally declaring independence from Great Britain. Two days later, the Declaration of Independence was adopted. Victory in the 1775–1783 Revolutionary War brought international recognition of U.S. sovereignty.
The U.S. expanded westward through purchase, settlement and conquest of European- and Indigenous-controlled territory. As more states joined the Union, a sectional division over slavery led 11 Southern states to secede and form the Confederate States of America, fighting the Union in the Civil War of 1861–1865. With the United States' victory and reunification, slavery was abolished. By 1900, the U.S. emerged as a great power, a status solidified after its involvement in World War I. Following Japan's attack on Pearl Harbor in 1941, it entered World War II on the Allied side. The war's aftermath left the U.S. and the Soviet Union as rival superpowers, vying for geopolitical dominance during the Cold War. The Soviet Union's collapse in 1991 left the U.S. as the world's sole superpower.
The U.S. federal government was established as a republic under the United States Constitution, with executive authority vested in a president. The Constitution created a separation of powers among legislative, executive, and judicial branches. The Congress is 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). Federalism grants substantial autonomy to the 50 states.
A developed country, the U.S. ranks high in economic competitiveness, innovation, and higher education. Accounting for over a quarter of nominal global GDP, its economy has been the world's largest since about 1890. It is the wealthiest country, with the highest disposable household income per capita among OECD members, though its wealth inequality is highly pronounced. Shaped by centuries of immigration, the culture of the U.S. is diverse and globally influential. Making up nearly a third of global military spending, the U.S. is widely considered to have the most powerful armed forces in the world. A member of numerous international organizations, it plays a major role in global political, cultural, economic, and military affairs.
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