The Collaborative International Dictionary of English v.0.48:

Foe \Foe\ (f[=o]), n. [OE. fo, fa, AS. f[=a]h hostile; prob.
   akin to E. fiend. [root]81. See Fiend, and cf. Feud a
   quarrel.]
   [1913 Webster]
   1. One who entertains personal enmity, hatred, grudge, or
      malice, against another; an enemy.
      [1913 Webster]

            A man's foes shall be they of his own household.
                                                  --Matt. x. 36
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   2. An enemy in war; a hostile army.
      [1913 Webster]

   3. One who opposes on principle; an opponent; an adversary;
      an ill-wisher; as, a foe to religion.
      [1913 Webster]

            A foe to received doctrines.          --I. Watts
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The Collaborative International Dictionary of English v.0.48:

Foe \Foe\, v. t.
   To treat as an enemy. [Obs.] --Spenser.
   [1913 Webster]

WordNet (r) 3.0 (2006):

foe
    n 1: an armed adversary (especially a member of an opposing
         military force); "a soldier must be prepared to kill his
         enemies" [syn: enemy, foe, foeman, opposition]
    2: a personal enemy; "they had been political foes for years"
       [syn: foe, enemy] [ant: ally, friend]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "foe":
   adversary, antagonist, archenemy, assailant, bitter enemy,
   combatant, devil, enemy, foeman, muddlement, open enemy, opponent,
   opposing party, opposite camp, public enemy, sworn enemy,
   the loyal opposition, the opposition