The Collaborative International Dictionary of English v.0.48:

War \War\, n. [OE. & AS. werre; akin to OHG. werra scandal,
   quarrel, sedition, werran to confound, mix, D. warren, G.
   wirren, verwirren, to embroil, confound, disturb, and perhaps
   to E. worse; cf. OF. werre war, F. querre, of Teutonic
   origin. Cf. Guerrilla, Warrior.]
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   1. A contest between nations or states, carried on by force,
      whether for defence, for revenging insults and redressing
      wrongs, for the extension of commerce, for the acquisition
      of territory, for obtaining and establishing the
      superiority and dominion of one over the other, or for any
      other purpose; armed conflict of sovereign powers;
      declared and open hostilities.
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            Men will ever distinguish war from mere bloodshed.
                                                  --F. W.
                                                  Robertson.
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   Note: As war is the contest of nations or states, it always
         implies that such contest is authorized by the monarch
         or the sovereign power of the nation. A war begun by
         attacking another nation, is called an offensive war,
         and such attack is aggressive. War undertaken to repel
         invasion, or the attacks of an enemy, is called
         defensive.
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   2. (Law) A condition of belligerency to be maintained by
      physical force. In this sense, levying war against the
      sovereign authority is treason.
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   3. Instruments of war. [Poetic]
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            His complement of stores, and total war. --Prior.
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   4. Forces; army. [Poetic]
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            On their embattled ranks the waves return,
            And overwhelm their war.              --Milton.
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   5. The profession of arms; the art of war.
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            Thou art but a youth, and he is a man of war from
            his youth.                            --1 Sam. xvii.
                                                  33.
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   6. a state of opposition or contest; an act of opposition; an
      inimical contest, act, or action; enmity; hostility.
      "Raised impious war in heaven." --Milton.
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            The words of his mouth were smoother than butter,
            but war was in his heart.             --Ps. lv. 21.
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   Civil war, a war between different sections or parties of
      the same country or nation.

   Holy war. See under Holy.

   Man of war. (Naut.) See in the Vocabulary.

   Public war, a war between independent sovereign states.

   War cry, a cry or signal used in war; as, the Indian war
      cry.

   War dance, a dance among savages preliminary to going to
      war. Among the North American Indians, it is begun by some
      distinguished chief, and whoever joins in it thereby
      enlists as one of the party engaged in a warlike
      excursion. --Schoolcraft.

   War field, a field of war or battle.

   War horse, a horse used in war; the horse of a cavalry
      soldier; especially, a strong, powerful, spirited horse
      for military service; a charger.

   War paint, paint put on the face and other parts of the
      body by savages, as a token of going to war. "Wash the war
      paint from your faces." --Longfellow.

   War song, a song of or pertaining to war; especially, among
      the American Indians, a song at the war dance, full of
      incitements to military ardor.

   War whoop, a war cry, especially that uttered by the
      American Indians.
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WordNet (r) 3.0 (2006):

war whoop
    n 1: a yell intended to rally a group of soldiers in battle
         [syn: war cry, war whoop, rallying cry, battle cry]