The Collaborative International Dictionary of English v.0.48:

Combativeness \Com"bat*ive*ness\, n.
   1. The quality of being combative; propensity to contend or
      to quarrel.
      [1913 Webster]

   2. (Phren.) A cranial development supposed to indicate a
      combative disposition.
      [1913 Webster]

WordNet (r) 3.0 (2006):

combativeness
    n 1: a militant aggressiveness [syn: combativeness,
         militance, militancy]