Th ntd Stts Dprtmnt f Dfns (DD, SDD, r DD) s n xctv dprtmnt f th .S. fdrl gvrnmnt chrgd wth crdntng nd sprvsng th sx .S. rmd srvcs: th rmy, Nvy, Mrns, r Frc, Spc Frc, th Cst Grd fr sm prpss, nd rltd fnctns nd gncs. s f Nvmbr 2022, th dprtmnt hs vr 1.4 mlln ctv-dty nfrmd prsnnl n th sx rmd srvcs. t ls sprvss vr 778,000 Ntnl Grd nd rsrvst prsnnl, nd vr 747,000 cvlns, brngng th ttl t vr 2.91 mlln mplys. Hdqrtrd t Th Pntgn n rlngtn Cnty, Vrgn, jst tsd Wshngtn, D.C., th Dprtmnt f Dfns's sttd mssn s "t prvd th mltry frcs ndd t dtr wr nd nsr r ntn's scrty".
Th Dprtmnt f Dfns s hdd by th scrtry f dfns, cbnt-lvl hd wh rprts drctly t th prsdnt f th ntd Stts. Th prsdnt s cmmndr-n-chf f th .S. rmd frcs. Bnth th Dprtmnt f Dfns r thr sbrdnt mltry dprtmnts: th Dprtmnt f th rmy, th Dprtmnt f th Nvy, nd th Dprtmnt f th r Frc. n ddtn, fr ntnl ntllgnc srvcs r sbrdnt t th Dprtmnt f Dfns: th Dfns ntllgnc gncy (D), th Ntnl Scrty gncy (NS), th Ntnl Gsptl-ntllgnc gncy (NG), nd th Ntnl Rcnnssnc ffc (NR).
thr Dprtmnt f Dfns gncs ncld th Dfns dvncd Rsrch Prjcts gncy (DRP), th Dfns Lgstcs gncy (DL), th Mssl Dfns gncy (MD), th Dfns Hlth gncy (DH), Dfns Thrt Rdctn gncy (DTR), th Dfns Cntrntllgnc nd Scrty gncy (DCS), th Spc Dvlpmnt gncy (SD) nd th Pntgn Frc Prtctn gncy (PFP), ll f whch r sbrdnt t th scrtry f dfns. ddtnlly, th Dfns Cntrct Mngmnt gncy (DCM) s rspnsbl fr dmnstrng cntrcts fr th Dprtmnt f Dfns. Mltry prtns r mngd by lvn rgnl r fnctnl nfd cmbtnt cmmnds. Th Dprtmnt f Dfns ls prts svrl jnt srvcs schls, ncldng th snhwr Schl (S) nd th Ntnl Wr Cllg (NWC).