<P> The United States of America (USA), commonly known as the United States (U.S.) or America (/ əˈmɛrɪkə /), is a federal republic composed of 50 states, a federal district, five major self - governing territories, and various possessions . Forty - eight states and the federal district are contiguous and located in North America between Canada and Mexico . The state of Alaska is in the northwest corner of North America, bordered by Canada to the east and across the Bering Strait from Russia to the west . The state of Hawaii is an archipelago in the mid-Pacific Ocean . The U.S. territories are scattered about the Pacific Ocean and the Caribbean Sea, stretching across nine official time zones . The extremely diverse geography, climate and wildlife of the United States make it one of the world's 17 megadiverse countries . </P> <P> At 3.8 million square miles (9.8 million km) and with over 324 million people, the United States is the world's third - or fourth - largest country by total area and the third-most populous . The capital is Washington, D.C., and the largest city is New York City; twelve other major metropolitan areas--each with at least 4.5 million inhabitants--are Los Angeles, Chicago, Dallas, Houston, Washington, D.C., Philadelphia, Miami, Atlanta, Boston, San Francisco, Phoenix, and Riverside--San Bernardino . </P> <P> Paleo - Indians migrated from Asia to the North American mainland at least 15,000 years ago . European colonization began in the 16th century . The United States emerged from 13 British colonies along the East Coast . Numerous disputes between Great Britain and the colonies following the Seven Years' War led to the American Revolution, which began in 1775, and the subsequent Declaration of Independence in 1776 . The war ended in 1783 with the United States becoming the first country to gain independence from a European power . The current constitution was adopted in 1788, with the first ten amendments, collectively named the Bill of Rights, being ratified in 1791 to guarantee many fundamental civil liberties . The United States embarked on a vigorous expansion across North America throughout the 19th century, acquiring new territories, displacing Native American tribes, and gradually admitting new states until it spanned the continent by 1848 . During the second half of the 19th century, the American Civil War led to the end of legal slavery in the country . By the end of that century, the United States extended into the Pacific Ocean, and its economy, driven in large part by the Industrial Revolution, began to soar . The Spanish--American War and World War I confirmed the country's status as a global military power . The United States emerged from World War II as a global superpower, the first country to develop nuclear weapons, the only country to use them in warfare, and a permanent member of the United Nations Security Council . The end of the Cold War and the dissolution of the Soviet Union in 1991 left the United States as the world's sole superpower . </P> <P> The U.S. is a founding member of the United Nations, World Bank, International Monetary Fund, Organization of American States (OAS), and other international organizations . The United States is a highly developed country, with the world's largest economy by nominal GDP and second - largest economy by PPP, accounting for approximately a quarter of global GDP . The U.S. economy is the fastest - growing in the Americas and is largely post-industrial, characterized by the dominance of services and knowledge - based activities, although the manufacturing sector remains the second - largest in the world . Though its population is only 4.3% of the world total, Americans hold 33.2% of the total wealth in the world, the largest share of global wealth concentrated in a single country . The United States ranks among the highest nations in several measures of socioeconomic performance, including average wage, human development, per capita GDP, and productivity per person . The U.S. is the foremost military power in the world, making up a third of global military spending . It is also a global leader in science and technology . </P>

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