Foe, n.
Etymology: [OE. fo, fa, AS. fh hostile; prob. akin to E. fiend. sq. root81.]
See Fiend, and cf. Feud a quarrel.
1. One who entertains personal enmity, hatred, grudge, or malice, against another; an enemy. A man's foes shall be they of his own household. Matt. x. 36
2. An enemy in war; a hostile army.
3. One who opposes on principle; an opponent; an adversary; an ill- wisher; as, a foe to religion. A foe to received doctrines. I. Watts
1 other Webster entry under this spelling
Foe, v. t.
To treat as an enemy. [Obs.] Spenser.
Shades of meaning
Fernald has no entry of its own for foe; he lists it under enemy.
Where he writes about it:
An enemy in private life is one who is moved by hostile feeling with active disposition to injure; but in military language all who fight on the opposite side are called enemies or collectively "the enemy," where no personal animosity may be implied; foe, which is rather a poetical and literary word, implies intensely hostile spirit and purpose. — from enemy