McNamara, himself, provided the perfect synopsis of this lesson (emphasis added):
What makes us omniscient? Have we a record of omniscience? We are the strongest nation in the world today. I do not believe that we should ever apply that economic, political, and military power unilaterally. If we had followed that rule in Vietnam, we wouldn’t have been there. None of our allies supported us. Not Japan, not Germany, not Britain or France. If we can’t persuade nations with comparable values of the merit of our cause, we’d better reexamine our reasoning.