I understand your "rotation" comment (that worked for a while during WWII until it became impractical to move entire units in and out of Europe, but slots did need to be filled), but in Viet Nam it was more the fact that they were often fighting in jungles so think you couldn't see what was a mere 10 fee ahead of you, night or day, and you couldn't tell if the Vietnamese man or woman or kid standing next to you in a village was friendly, or a member of the Viet Cong. In one instance, six graduates of the same West Point were assigned to the same unit at the same time and four were immediately killed in a jungle ambush. My 'take away,' at least, having watched the first four or five programs that covered just up to the time I got there.