The Collaborative International Dictionary of English v.0.48:

Ruse \Ruse\, n. [F., fr. OF. re["u]ser, rehuser, to turn aside,
   to shuffle, retreat, fr. L. recusare to refuse; pref. re-
   again + causa cause. See Cause, and cf. Recusant.]
   An artifice; trick; stratagem; wile; fraud; deceit.
   [1913 Webster]

   Ruse de guerre[F.], a stratagem of war.
      [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

RUSE DE GUERRE. Literally a trick in war; a stratagem. It is said to be
lawful among belligerents, provided it does not involve treachery and
falsehood. Grot. Droit de la Guerre, liv. 3, c. 1, Sec. 9.