If the army were trying out drugs on the platoon as we were told, why were the army so surprised to hear they all killed each other? Test number 1 of the drug was on monkeys and they all killed each other. Test number 2 was carried out on POWs and they all killed each other. Why on earth would they suddenly expect their test subjects to become picky and only attack "the enemy"?
By 1971 most of the ground fighting was being done by South Viets anyway---they had no problem going 'third world scum' on the NVA; the remaining US combat ground troops either had no problem fighting (SpecOps people like SEALs & SOG) or did played 'let's not be the last guy to die in Vietnam so don't look for trouble'(The remnants of the 1st Marine Div, 101st & the 'Americal'); any way the whole 'use drugs to make them fierce' never made sense to me.
On a related topic, guys like SOG DID use 'speed' sometimes--when trapped in Laos & being chased by half the NVA up & down mountains covered with triple canopy jungle, you really wanted to be able to 'run like hell' & didn't want to suddenly 'stop from exhaustion'.
Why can't you wretched prey creatures understand that the Universe doesn't owe you anything!?
I always think of Spud from Trainspotting when I think of speed. Your description has left me with a disturbing image of an army of Spuds in Vietnam. In honesty, I'd take the speed given the alternative.
I'm pretty sure the US army still give out amphetamines in some form. Also it was more used to keep people awake under long operations than to increase their stamina.
We don't know for sure that they were even given anything. Don't forget that all of that discussion was going on in his mind after he had been wounded and was dying. And if they really were drugged and the scenes we saw of his platoon killing each other were real, we still don't know how many times they tested the drugs and what the results were because, again, he was dreaming when they were told about the drug tests.
I always assumed it was in the "killer" weed the other guys smoked. Jacob doesn't smoke it, and most of the other guys do. Plus, it's mentioned a number of times throughout the movie.
The guy said they gave them an infinitesimal dose, so the assumption is that they thought it wouldn't make them go crazy like the other test subjects, just become more aggressive. Of course, they were wrong.