Rearguard

A rearguard is that part of a military force that protects it from attack from the rear, either during an advance or withdrawal. The term can also be used to describe forces protecting lines, such as communication lines, behind an army. Even more generally, a rearguard action may refer idiomatically to an attempt at preventing something though it is likely too late to be prevented; this idiomatic meaning may apply in either a military or non-military context.

The term rearguard (also rereward, rearward) originates from the medieval custom of dividing an army into three battles or wards; Van, Main or Middle and Rear. The Rear Ward usually followed the other wards on the march and during a battle usually formed the rearmost of the three if deployed in column or the left-hand ward if deployed in line.

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. This definition was later extended to highly mobile infantry as well as mechanized or armored forces.

Narrowly defined, a rearguard is a covering detachment that protects the retreating main ground force element (main body), or column, and is charged with executing defensive or retrograde movements between the main body and the enemy to prevent the latter from attacking or interfering with the movement of the main body.

A more expansive definition of the rearguard arose during the large-scale struggles between nation-states during the First and Second World Wars. In this respect, a rearguard is 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 interfering with advancing enemy forces in order to gain time for the remainder to regroup and reorganize. Rearguard actions may be undertaken in a number of ways, either in defense, such as by defending strongpoints or tactically important terrain, or by pre-emptively assaulting the enemy as he prepares his own offensive operations with a spoiling attack.

Examples of rearguard actions are these two:

Another contemporary example is the rearguard action fought by small units of the Serbian Army to protect the Serbian Army, its royal family, and Serbian refugees from advancing forces of the Central Powers during their retreat through Albania and Montenegro in 1915-1916. The nature of combat in rearguard actions involving combat between armies of nation-states is typically desperate and vicious, and rearguard troops may be called upon to incur heavy casualties or even to sacrifice all of their combat strength and personnel for the benefit of the withdrawing forces.

...
Wikipedia