Your partially stated assumption here is that soldiers stopped using drugs because of the punishments. I think this is a case of post hoc fallacy. Yes, punishme

Your partially stated assumption here is that soldiers stopped using drugs because of the punishments. I think this is a case of post hoc fallacy. Yes, punishments create a disincentive for some behavior, but only in the case of rational actors who have the means to act on that motive. Some soldiers who left Vietnam had the necessary support systems to overcome addiction or were never addicted in the first place or weren't in an environment where those drugs were available at home. Others did not and stayed addicted, even when they came home. Heavy penalties don't necessarily cause a proportionally smaller addiction problem. They just punish heavier. The only rational path to reducing drug addiction is to improve the conditions that cause drug addictions. Very few people become drug addicts for no reason.