The Vietnam War was really the birth of armed assault helicopters. Helicopters had been used mainly for transportation and reconnaissance in previous conflicts but Vietnam is where they began to get outfitted with weapons. Vietnam also lead to the creation of the attack helicopter, the first helicopter with combat as its only priority. The helicopter proved to be a vital tool in Vietnam, a fact that was noticed by the US military especially. One of the most modern and powerful helicopters of today, the AH-64 Apache, is a direct result of the Vietnam War.

Vast mountainous, jungle terrain with limited areas for traditional runways..

You need a quick way to move troops, supplies, munitions, small artillery pieces and provide close air support...

Arming helicopters is the obvious choice.