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 km2) 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 descendent and isolated Native American population was greatly reduced by European contact, primarily by disease after European colonization occurred, beginning in the 1500s. 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 with direct help from France and several other European powers, 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 amendments guaranteeing many fundamental civil rights and freedoms, was ratified in 1791.
Through the 19th century, the United States embarked on a vigorous program of expansion across North America. It displaced native tribes, acquiring the Louisiana territory from France and Florida from Spain; annexed the Republic of Texas in 1845, leading to war in which it conquered a large area of Mexico; and purchased Alaska from Russia in 1867. During the early territorial expansion, significant disputes between the agrarian slave-holding South and industrial North led to the American Civil War. The North's victory reestablished the Union, leading to the ratification of the Thirteenth Amendment to the United States Constitution and ending legalized slavery in the United States. The Plains Indian Wars relocated remaining tribes onto confined reservations, a Congressional Resolution annexed the Republic of Hawaii, then the treaty ending the Spanish-American War ceded Puerto Rico, Guam, as well as the Philippines (which later became independent). By the end of the nineteenth century, the American national economy was the world's largest.
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 the first country with nuclear weapons and a permanent member of the United Nations Security Council. The end of the Cold War and the dissolution of the Soviet Union left the United States as the sole superpower. The U.S. economy is the world's largest national economy, with an estimated 2011 GDP of $15.1 trillion (22% of nominal global GDP and over 19% of global GDP at purchasing-power parity). Per capita income is the world's sixth-highest. The country accounts for 41% of global military spending, and is a leading economic, political, and cultural force in the world.