rear guard

[Middle English reregarde, from Old French : rere, backward (from Latin retrō) + guarde, guard (from guarder, to defend; see guard).]

(DOD) 1. The rearmost elements of an advancing or a withdrawing force. It has the following functions: to protect the rear of a column from hostile forces; during the withdrawal, to delay the enemy; during the advance, to keep supply routes open. 2. Security detachment that a moving ground force details to the rear to keep it informed and covered. See also guard.