[@BingTheWing] Opium, and particularly heroin, was already a popular drug of choice in the 40s. Its popularity on the street doesn't see a massive decline until the early eighties when crack cocaine comes on the scene. Heroin in particular even survives the wave powdered cocaine made throughout the sixties and seventies. All the stories about the mob never dealing with opium/heroin and sticking to alcohol and cigarettes are incredulous at best: how do you think heroin and opium found their way into African-American and Latino neighborhoods with such ferocity? the mob. Sure, some old school dons didn't particularly like their guys getting involved because of the [i]potential[/i] heat, but its use on the street was such a new phenomena that it had twice as much street value as cigarettes and alcohol--and what the mob cares about over all else is money. So, yeah it was frowned upon but the leadership doesn't necessarily care so long as they get their cut every week.

Pertaining to the WWII, opium was used as a painkiller (and was used as the painkiller of choice in the nineteenth century as well; opium isn't novel in its use for wounded soldiers on the frontlines). Droves of U.S. soldiers quickly became addicted to the morphine inside it. So yes, opium was a well known and well abused drug by American troops.