I refused induction into the Army in 1968 because of my refusal to participate in the Vietnam War (or in any other war, for that matter). The "lessons" of Vietnam are very clear to me, and they can, in fact, be reduced to a single "lesson." No plural is needed. It is a "lesson" that I believe is generally applicable. Applicable to other wars; applicable to other situations; applicable to individuals; applicable to nations:
When you have made a mistake,
When you are going down the wrong road,
When you are going the wrong way,
You have to turn around.