Barrier troops, blocking units, or anti-retreat forces are formations of soldiers normally placed behind regular troops on a battle line to prevent panic or unauthorized withdrawal or retreat. Barrier troops may be utilized simply to raise the morale of front-line troops and for the purpose of constituting a reserve force, or they may be used to prevent unauthorized withdrawal of soldiers from the battlefield by any means, including indiscriminate killing. As troops guarding other units are obviously not available to fight the enemy they are a costly, often desperate, measure.