The Collaborative International Dictionary of English v.0.48:

Disunite \Dis`u*nite"\, v. t. [imp. & p. p. Disunited; p. pr.
   & vb. n. Disuniting.]
   1. To destroy the union of; to divide; to part; to sever; to
      disjoin; to sunder; to separate; as, to disunite particles
      of matter.
      [1913 Webster]

   2. To alienate in spirit; to break the concord of.
      [1913 Webster]

            Go on both in hand, O nations, never be disunited,
            be the praise . . . of all posterity! --Milton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

disunited
    adj 1: having been divided; having the unity destroyed;
           "Congress...gave the impression of...a confusing sum of
           disconnected local forces"-Samuel Lubell; "a league of
           disunited nations"- E.B.White; "a fragmented coalition";
           "a split group" [syn: disconnected, disunited,
           fragmented, split]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "disunited":
   alienated, detached, disaffected, disarticulated, disconnected,
   disengaged, disjoined, disjoint, disjointed, disjunct, dislocated,
   dispersed, divided, divorced, estranged, irreconcilable, isolated,
   removed, scattered, segregated, separated, sequestered, shut off,
   torn