The United States of America (USA), commonly known as the United States (U.S. or US) or America, is a country comprising 50 states, a federal district, five major self -governing territories, and various possessions. At 3.8 million square miles (9.8 million km2), the United States is the world 's third or fourth largest country by total area and is slightly smaller than the entire continent of Europe. With a population of over 327 million people, the U.S. is the third most populous country. The capital is Washington, D.C., and the most populous city is New York City. Most of the country is located contiguously in North America betweenCanada and Mexico.
Paleo-Indians migrated from Siberia to the North American mainland at least 12,000 years ago. European colonization began in the 16th century. The United States emerged from the thirteen British colonies established along the East Coast. Numerous disputes between Great Britain and the colonies led to the American Revolutionary War lasting between 1775 and 1783, leading to independence. The United States embarked on a vigorous expansion across North America throughout the 19th century, acquiring new territories, displacing Native Americans, and gradually admitting new states until spanning the continent by 1848.