United States

 The United States of America (USA or U.S.A.), commonly called the United States (US or U.S.) and colloquially as America, is a federal constitutional republicconsisting of fifty states and a federal district.[4]  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 Canadato the north and Mexico to the south. The state of Alaska is situated in the northwest portion of the continent, with Canada to its east and Russia to its west across theBering 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.[10]  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 colonieslocated 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 defeated Britain in the Revolutionary War, which became the first successful war of independence against a European empire.[11] [12]  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, 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, 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.[13]  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, thefirst 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 2012 GDP of $15.6 trillion – 19% of global GDP atpurchasing-power parity, as of 2011.[6] [14] [15]  The per capita GDP of the U.S. was the world's sixth-highest as of 2010.[6]  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,<sup class="reference" id="cite_ref-16" style="line-height:1em;">[16]  and is a leading economic, political, and cultural force in the world, as well as a leader in scientific research and technological innovation.<sup class="reference" id="cite_ref-17" style="line-height:1em;">[17] <sup class="reference" id="cite_ref-18" style="line-height:1em;">[18]