The United States of America (U.S.A. or USA), commonly called the United States (US or U.S.) and America, 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 around 315 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. The geography and climate of the United States is also extremely diverse and is home to a variety of species.
Paleoindians migrated from Asia to what is now the United States mainland around 15,000 years ago. After 1500, Old World diseases introduced by Europeans greatly reduced their populations. European colonization began around 1600 and came mostly from England. The United States emerged from thirteen British colonies located along the Atlantic seaboard, which developed their own economies and democratic political systems. Disputes between Great Britain and the American colonies led to the American Revolution. On July 4, 1776, delegates from the 13 colonies unanimously issued the Declaration of Independence, which established the United States of America. The new country with French aid defeated Britain in the Revolutionary War, which became the first successful war of independence against a European empire. The current Constitution was adopted on September 17, 1787; several Amendments were later added to the Constitution, modifying its effects but not changing the original text. The first ten amendments, collectively named the Bill of Rights, which guarantee many fundamental civil rights and freedoms, were ratified in 1791.
The United States embarked on a vigorous expansion across North America throughout the 19th century, displacing native tribes, acquiring new territories, and gradually admitting new states. The American Civil War ended legalized slavery in the United States. 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 a global power and 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 United States is a post-industrial developed country and has the world's largest national economy, with an estimated 2012 GDP of $15.6 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 economy is fueled by an abundance of natural resources, a well-developed infrastructure, and high productivity; and while its economy is considered post-industrial it continues to be one of the world's largest manufacturers. The country accounts for 41% of global military spending, and is a leading economic, political, and cultural force in the world, as well as a leader in scientific research and technological innovation.
How places in the United States are organized
All places in the United States
Further information on historical place organization in the United States