TheUnitedStatesofAmericaisacountryofthewesternhemisphere,comprisingfiftystatesandseveralterritories.Forty-eightcontiguousstateslieincentralNorthAmericabetweenthePacificandAtlanticOceans,boundedonlandbyCanadatothenorthandMexicotothesouth;AlaskaisinthenorthwestofthecontinentwithCanadatoitseast,andHawaiiisinthemid-Pacific.[2]TheUnitedStatesisafederalconstitutionalrepublicwithWashingtonDCitsfederalcapital.[3]

　　Atover3.7millionsquaremiles(over9.6millionkm²)andwithover300millionpeople,theUnitedStatesisthethirdorfourthlargestcountrybytotalareaandthirdlargestbypopulation.[4]Withagrossdomesticproduct(GDP)ofover$13trillion,theU.S.hasthelargestnationaleconomyintheworld.[5]GDPpercapitarankstheU.S.firstamonglargeeconomies.

　　Americansocietyistheproductoflarge-scaleimmigrationandishometoacomplexsocialstructure[6]aswellasawidearrayofhouseholdarrangements.[7]TheU.S.isoneoftheworld'smostethnicallyandsociallydiversenations.[8]

　　ThenationwasfoundedbythirteencoloniesdeclaringtheirindependencefromGreatBritainonJuly4,1776.Itadoptedthecurrentconstitution(whichhasbeenamendedseveraltimessubsequently)onSeptember17,1787.Thecountrygreatlyexpandedinterritorythroughoutthe19thcentury,acquiringfurtherterritoryfromtheUnitedKingdom,aswellaslandsfromFrance,Mexico,Spain,andRussia.WiththecollapseoftheSovietUnionin1991,itbecametheworld'ssoleremainingsuperpower,andisadeclarednuclearweaponsstate.TheUnitedStatescontinuestoexertdominanteconomic,political,culturalandmilitary