Very, very, very strong pot.
It was the synthetic, concentrated kind, and soldiers volunteered (out of patriotism, of course) to test the stuff. The studies began in the 1950s and ended in the early '70s, with disappointing results [source: Willing].
Researchers noted such effects as uncontrollable, hysterical laughter with no apparent cause; hallucinations that lasted for days; and soldiers just smirking for hours on end [source: Willing]. And while it seems such effects may indeed have hindered enemy ranks, the military was in the market for incapacitation, and researchers realized quickly that the initial "knockdown effect" of the drug wore off when the soldiers lay down and rested [source: Willing]. Again, seems like that would have slowed the enemy's progress, but ultimately the Army gave up on weaponizing weed.
And so laid-back, voraciously hungry warfare was not to be.