The Collaborative International Dictionary of English v.0.48:

War \War\, v. i. [imp. & p. p. Warred; p. pr. & vb. n.
   Warring.]
   1. To make war; to invade or attack a state or nation with
      force of arms; to carry on hostilities; to be in a state
      by violence.
      [1913 Webster]

            Rezin the king of Syria, and Pekah the son of
            Remaliah, king of Israel, went up toward Jerusalem
            to war against it.                    --Isa. vii. 1.
      [1913 Webster]

            Why should I war without the walls of Troy? --Shak.
      [1913 Webster]

            Our countrymen were warring on that day! --Byron.
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   2. To contend; to strive violently; to fight. "Lusts which
      war against the soul." --1 Pet. ii. 11.
      [1913 Webster]

WordNet (r) 3.0 (2006):

warring
    adj 1: engaged in war; "belligerent (or warring) nations" [syn:
           belligerent, militant, war-ridden, warring]