The Collaborative International Dictionary of English v.0.48:

Disunion \Dis*un"ion\, n. [Pref. dis- + union: cf. F.
   d['e]sunion.]
   1. The termination of union; separation; disjunction; as, the
      disunion of the body and the soul.
      [1913 Webster]

   2. A breach of concord and its effect; alienation.
      [1913 Webster]

            Such a disunion between the two houses as might much
            clou? the happiness of this kingdom.  --Clarendon.
      [1913 Webster]

   3. The termination or disruption of the union of the States
      forming the United States.
      [1913 Webster]

            I have not accustomed myself to hang over the
            precipice of disunion.                --D. Webster.
      [1913 Webster]

WordNet (r) 3.0 (2006):

disunion
    n 1: the termination or destruction of union [ant:
         conjugation, jointure, unification, union,
         uniting]

Moby Thesaurus II by Grady Ward, 1.0:

82 Moby Thesaurus words for "disunion":
   abstraction, alienation, antagonism, argumentation, breach,
   breach of friendship, break, clashing, cleavage, cleft, conflict,
   contention, contradiction, controversy, detachment, difference,
   disaccord, disaccordance, disaffection, disagreement,
   disarticulation, disassociation, disconnectedness, disconnection,
   discontinuity, discord, discordance, discordancy, discrepancy,
   disengagement, disfavor, disharmony, disjointing, disjunction,
   dislocation, disparity, disruption, dissension, dissent,
   dissidence, dissolution, dissonance, disunity, divergence,
   diversity, dividedness, division, divorce, divorcement,
   estrangement, faction, falling-out, inaccordance, incoherence,
   inequality, isolation, jarring, luxation, negation, open rupture,
   opposition, oppugnancy, parting, partition, recall of ambassadors,
   removal, repugnance, rift, rupture, schism, segmentation,
   separation, separatism, split, split-up, strife, subdivision,
   subtraction, unharmoniousness, variance, withdrawal, zoning