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.
      [1913 Webster]

   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]

Moby Thesaurus II by Grady Ward, 1.0:

88 Moby Thesaurus words for "warring":
   aggressive, ajar, all-out war, antagonistic, appeal to arms,
   armed combat, armed conflict, attack, battle, battling, bellicose,
   belligerence, belligerency, belligerent, bloodshed, bloodthirsty,
   bloody, bloody-minded, chauvinist, chauvinistic, clashing, combat,
   combative, conflicting, confused, contending, contentious,
   contestant, contesting, disputant, enemy, ferocious, fierce,
   fighting, full of fight, grating, harsh, hawkish, hostile,
   hostilities, hot war, inimical, jangling, jangly, jarring, jingo,
   jingoish, jingoist, jingoistic, jostling, la guerre, martial,
   might of arms, militant, militaristic, military,
   military operations, offensive, open hostilities, open war,
   pugnacious, quarrelsome, resort to arms, saber-rattling,
   sanguinary, sanguineous, savage, scrappy, shooting war,
   soldierlike, soldierly, state of war, striving, struggling,
   the sword, total war, trigger-happy, truculent, unfriendly,
   unpacific, unpeaceable, unpeaceful, war, warfare, warlike,
   warmaking, warmongering, wartime