The United States of America is a country of the western hemisphere,comprising fifty states and several territories.Forty-eight contiguous states lie in central North America between the Pacific and Atlantic Oceans,bounded on land by Canada to the north and Mexico to the south; Alaska is in the northwest of the continent with Canada to its east,and Hawaii is in the mid-Pacific.[2] The United States is a federal constitutional republic with Washington DC its federal capital.[3]
At over 3.7 million square miles (over 9.6 million km²) and with over 300 million people,the United States is the third or fourth largest country by total area and third largest by population.[4] With a gross domestic product (GDP) of over $13 trillion,the U.S.has the largest national economy in the world.[5] GDP per capita ranks the U.S.first among large economies.
American society is the product of large-scale immigration and is home to a complex social structure[6] as well as a wide array of household arrangements.[7] The U.S.is one of the world's most ethnically and socially diverse nations.[8]
The nation was founded by thirteen colonies declaring their independence from Great Britain on July 4,1776.It adopted the current constitution (which has been amended several times subsequently) on September 17,1787.The country greatly expanded in territory throughout the 19th century,acquiring further territory from the United Kingdom,as well as lands from France,Mexico,Spain,and Russia.With the collapse of the Soviet Union in 1991,it became the world's sole remaining superpower,and is a declared nuclear weapons state.The United States continues to exert dominant economic,political,cultural and military