The United States of America (commonly called the United States, the U.S., the USA, America, and the States) is a federal constitutional republic consisting of fifty states and a federal district. The country is situated mostly in central North America, where its forty-eight contiguous states and Washington, D.C., the capital district, lie between the Pacific and Atlantic Oceans, bordered by Canada to the north and Mexico to the south. The state of Alaska is in the northwest of the continent, with Canada to the east and Russia to the west across the Bering Strait. The state of Hawaii is an archipelago in the mid-Pacific. The country also possesses several territories in the Pacific and Caribbean. At 3.79 million square miles (9.83 million km) and with over 314 million people, the United States is the third- or fourth-largest country by total area, and the third-largest by both land area and population. It is one of the world's most ethnically diverse and multicultural nations, the product of large-scale immigration from many countries.
Paleoindians migrated from Asia to what is now the United States mainland around 15,000 years ago. The Native American population descendent from them was in turn greatly reduced, primarily by disease after European contact and exasperated by European colonization. The United States itself was initially derived from thirteen British colonies located along the Atlantic seaboard. On July 4, 1776, delegates to the Continental Congress issued the Declaration of Independence, which proclaimed their right to self-determination and establishment of a sovereign union. The rebellious states defeated the British Empire in the American Revolution, the first successful colonial war of independence. The current United States Constitution was adopted on September 17, 1787; its ratification the following year made the states part of a single republic with a stronger central government. The Bill of Rights, consisting of ten constitutional