The United States of America (USA or U.S.A.), commonly referred to as the United States (US or U.S.), America, or simply the States, is a federal republic consisting of 50 states and a federal district. The 48 contiguous states and the federal district of Washington, D.C. are in central North America between Canada and Mexico. The state of Alaska is the northwestern part of North America, west of Canada and east of Russia which is across the Bering Strait in Asia, and the state of Hawaii is an archipelago in the mid-North Pacific. The country also has five populated and nine unpopulated territories in the Pacific and the Caribbean.
At 3.79 million square miles (9.83 million km2) in total and with around 315 million people, the United States is the fourth-largest in total area, the fifth largest in the contiguous area and third in 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 U.S. is also extremely diverse, with deserts, plains, forests, and mountains that are home to a wide variety of wildlife.
Paleo-indians migrated from Asia to what is now the United States mainland around 12,000 years ago. European colonization began around 1600, mostly from England. The United States emerged from 13 British colonies located along the Atlantic seaboard. 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 American Revolutionary War, which ended with the recognition of independence of the United States from the Kingdom of Great Britain, was the first successful war of independence against a European colonial empire. The current Constitution was adopted on September 17, 1787; 27 Amendments have since been added to the Constitution. The first 10 amendments, collectively named the Bill of Rights, were ratified in 1791 and guarantee many fundamental civil rights and freedoms.
Driven by the doctrine of manifest destiny, the United States embarked on a vigorous expansion across North America throughout the 19th century. This involved 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 19th century, the United States extended into the Pacific Ocean, and its 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 superpower, 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 developed country and has the world's largest national economy, with an estimated 2013 GDP of $16.2 trillion –22% of global GDP at purchasing-power parity, as of 2011. The per capita GDP of the U.S. was the world's sixth-highest as of 2010. The US has the fourth most unequal income distribution among OECD nations, behind Chile, Mexico and Turkey. 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 39% of global military spending, being the foremost economic and military power, a prominent political and cultural force in the world, and a leader in scientific research and technological innovation.