The Collaborative International Dictionary of English v.0.48:

Belligerence \Bel*lig"er*ence\, Belligerency \Bel*lig"er*en*cy\,
   n.
   The quality of being belligerent.
   [1913 Webster]

   2. the act or state of being engaged in war or a warlike
      conflict; warfare.

   Syn: hostilities.
        [WordNet 1.5]

   3. an aggressively hostile or warlike attitude or nature; a
      readiness to fight or offend, with little or no
      provocation.
      [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

belligerence
    n 1: hostile or warlike attitude or nature [syn: belligerence,
         belligerency]
    2: a natural disposition to be hostile [syn: aggressiveness,
       belligerence, pugnacity]

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "belligerence":
   aggression, aggressiveness, all-out war, antagonism, antipathy,
   appeal to arms, argumentativeness, armed combat, armed conflict,
   attack, battle, bellicism, bellicosity, belligerency, bickering,
   bloodshed, chauvinism, clash, clashing, collision, combat,
   combativeness, conflict, contention, contentiousness,
   despitefulness, disputatiousness, dissension, dissent, dissidence,
   faction, factiousness, ferocity, fierceness, fight, fighting, flak,
   friction, hate, hatred, hostilities, hostility, hot war,
   infighting, irascibility, irritability, jingoism, la guerre,
   litigiousness, malevolence, malice, malignity, martialism,
   might of arms, militancy, militarism, military operations,
   open hostilities, open war, partisan spirit, partisanship,
   pugnaciousness, pugnacity, quarrelsomeness, repugnance,
   resort to arms, saber rattling, shooting war, shrewishness, spite,
   spitefulness, state of war, the sword, total war, truculence,
   unfriendliness, unpeacefulness, war, warfare, warmaking,
   warmongering, warpath, warring, wartime