How the heck would you calibrate the dosage?
You’d have to somehow broadly disseminate enormous quantities of a drug, widely enough and thoroughly enough that pretty much the entire military of the country accumulates a high enough dosage that they become so depressed that they don’t bother to fight when their country is invaded. These would, by the way, pretty much by definition, be mostly young adults, and among the fittest and healthiest individuals in the entire country.
Meanwhile, this dosage would have to somehow not cause severe adverse reactions in the entire rest of the population, including children, infants, the elderly, and the sick. And, by the way, psychotropic drugs often have wildly different effects in adolescents than they do in adults. And we have little idea of what effects most psychotropics have in young children, since that’s so hard to ethically study.
SSRIs are directly administered by medical professionals, in very precise doses, under carefully controlled conditions. Even then, it’s impossible to precisely predict how any given individual will react to any given dose. Some SSRIs will simply have no clinical effect on some individuals. Other SSRI regimes will produce clinically relevant results in some individuals only with careful monitoring and constant tweaking of dosages, along with monitoring of diet, exercise, and so forth.
JB99 references the use of anesthetic gas during a hostage rescue operation, the Moscow theater hostage siege of 2002, as “semi-successful”. In fact, some 200 of the 805 hostages were killed by the “non-lethal” incapacitating agent used.
Even putting aside how you would physically manage to covertly disseminate clinically relevant dosages of a psychotropic to an entire country’s population, the fact is that in the real world, there’s simply no way to calibrate the dosages everyone would receive. Some people would almost certainly receive several times the raw dosage as others; the difference could easily be one or more orders of magnitude. And people vary wildly in weight, metabolism, overall health, usage of other drugs which might interact with your chosen agent, and reaction to the agent itself.
JB99 also references Serenity. Frankly, that’s probably a fairly realistic outcome of such a scheme. Ok, you probably wouldn’t tranquilize 99% of the population to the point that they don’t even bother to breathe while turning 1% into cinematic serial killer mutant cannibals. But you would almost certainly kill a lot of people with overdoses and leave a lot of others with severe and probably irreparable psychological damage. What the Serenity scenario doesn’t include that the real world would would be a lot of people who, due to some combination of natural resistance and receiving a smaller dosage than average, would remain highly functional, would know something was going very wrong, and would be highly motivated to find out what that was and who was responsible. And take retaliatory action. Probably not face-eating, but still…