The United States of America (also referred to as the United
States, the U.S., the USA, or America) is a federal constitutional republic comprising fifty states and a federal district. The
country is situated mostly in central North America, where its forty-eight contiguous states and Washingto,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 Caribbean and Pacific.
At 3.79 million square miles (9.83 million km2) and with over 310
million people, the United States is the third or
fourth largest country by total area, and the third largest both by 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
countriesThe U.S. economy is the world's
largest national economy, with an estimated 2009 GDP of
$14.3 trillion (a quarter of nominal global GDP and a
fifth of global GDP at purchasing power parity.
Indigenous peoples of Asian origin as well as haplogroup X have inhabited what is now the mainland United States for many thousands of
years. This Native American population was greatly reduced by disease and warfare after European contact. The
Native Americans had belonged to various tribal groups and lived in small
villages and did not coalesce to form larger political units. Because they
lacked central organization, they were easily subjugated by outsiders.
The United States was founded by thirteen British colonies located along the Atlantic seaboard. On July 4,
1776, they issued the 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, 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 strong central government. The Bill of Rights, comprising ten constitutional amendments guaranteeing many fundamental civil rights and freedoms, was
ratified in 1791.
In the 19th century, the United States acquired land from France, Spain, the United Kingdom, Mexico, and Russia, and annexed the Republic of Texas and the Republic
of Hawaii. Disputes between the agrarian South and industrial
North over states'
rights and the expansion of the institution of slavery provoked
the American Civil
War of the 1860s. The North's victory prevented a permanent split of the
country and led to the end of legal
slavery in the United States. By the 1870s, the national economy was the
world's largest.The Spanish–American War and World War I confirmed the
country's status as a military power. It 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 country accounts for 40% of global military spending and is a leading economic, political, and cultural force in the
world
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