The United States of America (also referred to as the United States, the U.S., the USA, or America IPAc-en?m?r?k?) is a federal (federal government) constitutional republic comprising fifty states (U.S. state) and a federal district. The country is situated mostly in central North America, where its forty-eight contiguous states (Contiguous United States) and Washington, D.C., the capital district (capital districts and territories), lie between the Pacific (Pacific Ocean) 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 (Territories of the United States) in the Caribbean and Pacific.

At 3.79 million square miles (9.83 million km2) and with over 309 million people, the United States is the third or fourth (List of countries and outlying territories by total area) largest country by total area, and the third largest both by land area (List of countries and outlying territories by land area) and population (List of countries by population). It is one of the worlds most ethnically diverse (Multiethnic society) and multicultural (multiculturalism) nations, the product of large-scale immigration from many countries (immigration to the United States). The U.S. economy (Economy of the United States) is the worlds largest national economy, with an estimated 2009 GDP (gross domestic product) of $14.3 trillion (a quarter of nominal global GDP (List of countries by GDP (nominal)) and a fifth of global GDP at purchasing power parity).

Indigenous peoples (Indigenous peoples of the Americas) of Asian origin (Models of migration to the New World) have inhabited what is now the mainland United States for many thousands of years. This Native American (Native Americans in the United States) population was greatly reduced by disease and warfare after European contact (European colonization of the Americas). The United States was founded by thirteen British colonies (Thirteen Colonies) located along the Atlantic seaboard (East Coast of the United States). On July 4, 1776, they issued the Declaration of Independence (United States Declaration of Independence), which proclaimed their right to self-determination and their establishment of a cooperative union. The rebellious states defeated the British Empire in the American Revolution (American Revolutionary War), the first successful colonial war of independence (History of colonialism). 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 strong central government. The Bill of Rights (United States Bill of Rights), comprising ten constitutional amendments (List of amendments to the United States Constitution) guaranteeing many fundamental civil rights and freedoms (natural rights), was ratified in 1791.

In the 19th century, the United States acquired land from France (Louisiana Purchase), Spain (Adams-On�s Treaty), the United Kingdom (Oregon Country), Mexico (Mexican-American War), and Russia (Alaska Purchase), and annexed (Annexation) the Republic of Texas and the Republic of Hawaii. Disputes between the agrarian South (Southern United States) and industrial North (Northern United States) over states rights and the expansion of the institution of slavery (slavery in the United States) provoked the American Civil War of the 1860s. The Norths victory prevented a permanent split of the country and led to the end of legal slavery (Thirteenth Amendment to the United States Constitution) in the United States. By the 1870s, the national economy was the worlds largest. The Spanish-American War and World War I confirmed the countrys status as a military power. It emerged from World War II as the first country with nuclear weapons (Nuclear weapons and the United States) and a permanent member of the United Nations Security Council. The end of the Cold War and the dissolution of the Soviet Union (History of the Soviet Union (1985-1991)) left the United States as the sole superpower. The country accounts for two-fifths of global military spending (List of countries by military expenditures) and is a leading economic, political, and cultural force in the world.