From the U.S. Army's The Big Picture television series, 1950-1975.
"In Vietnam, the enemy is called "Charlie." He maybe any place and anyone, man, woman or child. For neither the regular North Vietnamese regular army nor the sinister Viet Cong fights according to what might be called the rules of modern warfare. Bold, tough and crafty, theirs is the strategy of the guerilla. To avoid open conflict and fight from ambush, or to infiltrate civilian populations and spread confusion and terror by murder, mine or booby trap. The only effective way to counter these guerilla tactics is to beat Charlie to his punch, seek out and pursue him before he can put his plans into operation. Thus, in Vietnam when intelligence sources indicate the that enemy is preparing a build-up, our forces move swiftly to catch the Viet Cong off guard and frustrate their plans. This is done by means of a strike operation. Strike operations are swift moving, offensive actions in which the guerilla rather than terrain is the target. Because guerilla forces are constantly moving and shifting, planning for a strike operation must be flexible from the top down to the squad."--Transcript.