Rearguard

[2] The term rearguard (also rereward, rearward) comes from the Old French reregarde,[3] i.e. "the guard which is behind", originating with the medieval custom of dividing an army into three battles or wards; Van, Main (or Middle) and Rear.

The commonly accepted definition of a rearguard in military tactics was largely established in the battles of the late 19th century.

Before the mechanization of troop formations, most rearguard tactics originally contemplated the use of cavalry forces.

In this context, a rearguard can be a minor unit of regular or irregular troops that protect the withdrawal of larger numbers of personnel (military or civilian) during a retreat, by blocking, defending, delaying, or otherwise interfering with enemy forces in order to gain time for the remainder to regroup or reorganize.

[2] An example of a famous rearguard action outside the military context is the effort by Roman emperor Julian around 362 A.D. to restore Paganism as the state religion instead of Christianity.