TheUnitedStatesofAmerica(alsocalledtheUnitedStates,theU.S.,theUSA,America,andtheStates)isafederalconstitutionalrepubliccomprisingfiftystatesandafederaldistrict.ThecountryissituatedmostlyincentralNorthAmerica,whereitsforty-eightcontiguousstatesandWashington,D.C.,thecapitaldistrict,liebetweenthePacificandAtlanticOceans,borderedbyCanadatothenorthandMexicotothesouth.ThestateofAlaskaisinthenorthwestofthecontinent,withCanadatotheeastandRussiatothewestacrosstheBeringStrait.ThestateofHawaiiisanarchipelagointhemid-Pacific.ThecountryalsopossessesseveralterritoriesinthePacificandCaribbean. 　　At3.79millionsquaremiles(9.83millionkm2)andwithover312millionpeople,theUnitedStatesisthethirdorfourthlargestcountrybytotalarea,andthethirdlargestbybothlandareaandpopulation.Itisoneoftheworld'smostethnicallydiverseandmulticulturalnations,theproductoflarge-scaleimmigrationfrommanycountries.TheU.S.economyistheworld'slargestnationaleconomy,withanestimated2010GDPof$14.53trillion(23%ofnominalglobalGDPandover19%ofglobalGDPatpurchasing-powerparity). 　　IndigenouspeoplesdescendedfromforebearswhomigratedfromAsiahaveinhabitedwhatisnowthemainlandUnitedStatesformanythousandsofyears.ThisNativeAmericanpopulationwasgreatlyreducedbydiseaseandwarfareafterEuropeancontact.TheUnitedStateswasfoundedbythirteenBritishcolonieslocatedalongtheAtlanticseaboard.OnJuly4,1776,theyissuedtheDeclarationofIndependence,whichproclaimedtheirrighttoself-determinationandtheirestablishmentofacooperativeunion.TherebelliousstatesdefeatedtheBritishEmpireintheAmericanRevolution,thefirstsuccessfulcolonialwarofindependence.ThecurrentUnitedStatesConstitutionwasadoptedonSeptember17,1787;itsratificationthefollowingyearmadethestatespartofasinglerepublicwithastrongcentralgovernment.TheBillofRights,comprisingtenconstitutionalamendmentsguaranteeingmanyfundamentalcivilrightsandfreedoms,wasratifiedin1791. 　　Throughthe19thcentury,theUnitedStatesdisplacednativetribes,acquiredtheLouisianaterritoryfromFrance,FloridafromSpain,partoftheOregonCountryfromtheUnitedKingdom,AltaCaliforniaandNewMexicofromMexico,andAlaskafromRussia,andannexedtheRepublicofTexasandtheRepublicofHawaii.DisputesbetweentheagrarianSouthandindustrialNorthovertheexpansionoftheinstitutionofslaveryandstates'rightsprovokedtheCivilWarofthe1860s.TheNorth'svictorypreventedapermanentsplitofthecountryandledtotheendoflegalslaveryintheUnitedStates.Bythe1870s,itsnationaleconomywastheworld'slargest.TheSpanish–AmericanWarandWorldWarIconfirmedthecountry'sstatusasamilitarypower.ItemergedfromWorldWarIIasthefirstcountrywithnuclearweaponsandapermanentmemberoftheUnitedNationsSecurityCouncil.TheendoftheColdWarandthedissolutionoftheSovietUnionlefttheUnitedStatesasthesolesuperpower.Thecountryaccountsfor41%ofglobalmilitaryspending,anditisaleadingeconomic,political,andculturalforceintheworld.