rear guard

thumb|A United States Marine Corps|United States Marine providing rear security to his unit during a simulated [[patrol in 2009]] A rearguard or rear security is a 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.

==Origins== The term rearguard (also rereward, rearward) comes from the Old French reregarde, 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 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.