The United States of 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 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 its east and Russia to  the west across the Bering Strait, and the state of Hawaii is in the mid-Pacific. The United States also possesses several territories,  or insular areas, scattered around the Caribbean  and Pacific. 
	        
	         
	          At 3.79 million square miles (9.83  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 land area and by  population. The United    States is one of the world's most ethnically  diverse nations, the product of large-scale immigration from many countries. The  U.S.  economy is the largest national economy in the world, with a nominal 2006 gross  domestic product (GDP) of more than US$13 trillion (over 19% of the world total  based on purchasing power parity).  
	         
	          The nation was founded by thirteen  colonies of Great Britain  located along the Atlantic seaboard. Proclaiming themselves "states,"  they issued the Declaration of Independence on July 4, 1776. The rebellious  states defeated Great    Britain in the American Revolutionary War,  the first successful colonial war of independence. A federal convention adopted  the current United States Constitution on September 17, 1787; its ratification  the following year made the states part of a single republic. The Bill of Rights,  comprising ten constitutional amendments, was ratified in 1791. 
	          In the nineteenth century, the United States acquired land from France, Spain,  Great Britain, 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 slavery in the United States. The Spanish-American  War and World War I confirmed the nation's status as a military power. In 1945,  the United States  emerged from World War II as the first country with nuclear weapons, a  permanent member of the United Nations Security Council, and a founding member  of NATO. In the post–Cold War era, the United States is the only remaining superpower—accounting  for approximately 50% of global military spending—and a dominant economic,  political, and cultural force in the world 
	          
	          
	         
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