rearguard
Rearguard (noun) means the part of an army furthest back, especially soldiers guarding the rear of troops as they retreat.
Etymology
Came into English from Middle English rier garde, which came from Anglo-Norman reregard, reregarde, rereguarde and Middle French rieregarde, in turn from Old French rereguarde (attested in The Song of Roland, c. 1150), formed from rere (“after; behind”) and guarde (“guard”). Taken at face value it can be broken down as rear + guard, and it is a piecewise doublet of retrogarde.