A rear guard or 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 of communication behind an army.
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.
Contemporary usage 
The commonly-accepted definition of a rear guard in military tactics was largely established in the battles of the late 19th century. Before the mechanization of troop formations, most rear guard 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 rear guard 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 rear guard arose during the large-scale struggles between nation-states during the First and Second World Wars. In this respect, a rear guard 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. Rear guard 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. One example of a contemporary rear guard action is that 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 rear guard actions involving combat between armies of nation-states is typically desperate and vicious, and rear guard 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.
German blitzkrieg 
In the inter-war period German commanders (in particular Heinz Guderian) developed the doctrine of blitzkrieg. In this military doctrine the rear guard troops (mainly dismounted infantry) were tasked to eliminate the remaining enemy groups after Panzer and motorized troops had broken through the enemy positions.
Soviet doctrine 
During and after World War II the Soviets developed the doctrine of the echelons, influenced by blitzkrieg. Instead of sending all the troops to the attack, they divided the force in various parts according to the mission: for example, in a division-launched attack, one regiment would overrun enemy defenses, the second exploited the breach in enemy lines, and the third, the rear guard, being less highly trained, were assigned to mop up the enemy pockets of resistance and guard the supply lines.
To counter Soviet maneuvre groups, NATO created a number of quickly-deployable units (the bulk of NATO special forces) that were assigned roles that could be considered those of a rear guard.
See also 
- "rear guard." Military and Associated Words. US Department of Defense, 2003. 
- Rogers, Clifford (2007). Soldiers Lives through History: The Middle Ages. Westport: Greenwood. p. 73. ISBN 978-0-313-33350-7.
- Burnham, W.P. (Lt.) Manual of Guard Duty, U.S. Army, Syracuse, New York: C.P. Bardeen, Publisher (1893) pp. 92-95
- Bond, Paul Stanley (Lt. Col.) and Crouch, Edwin Hunter (1st Lt.), New York: The American Army and Navy Journal (1922) Tactics: the practical art of leading troops in war, pp. 247-253
- Headquarters, Dept. of the Army, 5-166: Spoiling Attack, Army Field Manual FM 3-90 (Tactics) (July 2001), p. 12-25
- Headquarters, Dept. of the Army, 5-166: Spoiling Attack, Army Field Manual FM 3-90 (Tactics) (July 2001), pp. 5-39, 5-40: A spoiling attack is a defensive attack, undertaken to preempt or seriously impair an enemy assault by attacking the enemy while the latter is in the process of assembly or preparation for offensive operations.
- Corey, Herbert, The Serbian Tragedy As I Saw It, Harper's Monthly Magazine (June 1917), p. 334
- Big Guns Blast Way in Serbia: Population Joins Retreat, The New York Times, 3 November 1915
- Frucht, Richard (ed.), Eastern Europe: an introduction to the people, lands, and culture, Vol. 3, ISBN 1-57607-800-0 (2005) p. 542: The Serbian rearguard actions allowed some 125,000-145,000 soldiers of Marshal Putnik's Serbian Army together with several thousand civilian refugees to reach Adriatic ports in Albania, where they were eventually evacuated, reorganized, and reequipped for the campaign in Salonika.
- Sebag-Montefiore, Hugh, Dunkirk: Fight to the Last Man, (1st ed.), Cambridge, MA: Harvard University Press, ISBN 978-0-674-02439-7 (2006), p. 233
- Bimberg, Edward L., World War II: A Tale of the French Foreign Legion, World War II Magazine (September 1997), p. 32: On 9 June 1940, the 97th Foreign Legion Divisional Reconnaissance Group, equipped with light armored cars, made a spoiling attack against German armored forces equipped with Pzkw Mk III tanks in order to protect the withdrawal of French Forces. In two consecutive assaults, the unit lost all its armored cars, incurring heavy casualties.