The Collaborative International Dictionary of English v.0.48:

hostilities \hostilities\ n. pl.
   1. a legal state created by a declaration of war and ended by
      official declaration, during which the international rules
      of war apply.

   Syn: war, state of war, hot war.
        [WordNet 1.5]

              We have showed ourselves generous adversaries . .
              . and have carried on even our hostilities with
              humanity.                           --Atterbury.
        [1913 Webster]

   2. acts of overt warfare.

   Syn: belligerency.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Hostility \Hos*til"i*ty\, n.; pl. Hostilities. [L. hostilitas:
   cf. F. hostilit['e].]
   1. State of being hostile; public or private enemy;
      unfriendliness; animosity.
      [1913 Webster]

            Hostility being thus suspended with France.
                                                  --Hayward.
      [1913 Webster]

   2. An act of an open enemy; a hostile deed; especially in the
      plural, acts of warfare; attacks of an enemy. See
      hostilities
      [1913 Webster]

            He who proceeds to wanton hostility, often provokes
            an enemy where he might have a friend. --Crabb.

   Syn: Animosity; enmity; opposition; violence; aggression;
        contention; warfare.
        [1913 Webster]

WordNet (r) 3.0 (2006):

hostilities
    n 1: fighting; acts of overt warfare; "the outbreak of
         hostilities" [syn: hostilities, belligerency]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "hostilities":
   all-out war, appeal to arms, armed combat, armed conflict, attack,
   battle, belligerence, belligerency, bloodshed, combat, fighting,
   hot war, la guerre, might of arms, military operations,
   open hostilities, open war, resort to arms, shooting war,
   state of war, the sword, total war, war, warfare, warmaking,
   warring, wartime