A free fire zone in U.S. military parlance is a fire control measure, used for coordination between adjacent combat units. The definition used in Vietnam by US troops may be found in field manual FM 6-20:

A specific designated area into which any weapon system may fire without additional coordination with the establishing headquarters.

See also rules of engagement. This article describes the military term of the rules of engagement. ...

Some civilians and others have said that U.S. military command in Vietnam had an established policy of designating as "free-fire zones":

• A battle area or combat zone in which anyone unidentified is considered an enemy combatant
• Areas in which soldiers were to shoot anyone moving around after curfew, without first making sure that they were hostile.

Since such encounters could result in the deaths of innocent civilians, it would have been a violation of the Geneva Convention to have had such a policy. The Geneva Conventions consist of treaties formulated in Geneva, Switzerland that set the standards for international law for humanitarian concerns. ...