From The Collaborative International Dictionary of English v.0.48:
Vanguard \Van"guard`\, n. [For vantguard, avantguard, F.
avant-garde; avant before, fore + garde guard. See Avant,
Ab-,Ante-, and Guard, and cf. Advance, Vamp, Van
of an army, Vaward.] (Mil.)
The troops who march in front of an army; the advance guard;