I'm not sure if it's been mentioned yet, but in seeing people bring up "The Mafia" and New World Order and things like that I figure I should post. Historically marijuana was criminalized, in the United States at least, during the early 1900's. The Southwest was seeing an influx of Mexican immigrants fleeing the Mexican Revolutution. Now of course these poor immigrants brought with them odd traditions and a cheap labor force, two things the American public has always felt threatened by. The local governments of these states, mainly Texas and New Mexico and the like saught a way to send these immigrants packing, but could not outright imprison them. Instead, they made illegal the plant that the immigrants smoked, marijuana. Marijuana itself is the Mexican term for Cannabis, and the government began calling it "marihuana" in order to give it a more devious and foreign sounding tone. These anti-marijuana laws were actually the first of the drug laws passed in America, and were to set a tone for the rest of the centuries policy toward drug use. (Previous drug laws were made against cocaine in order to arrest "crazed negros" who were accussed of "raping white women", while in reality they were just poor unemployed men who the government wanted nothing to do with).
Marijuana only became illegal nation wide after the drug spread away from the rich, cultured, bourgeoise elites and fell into the hands of the middle-class.
It remained illegal despite numerous scientific studies showing that it didn't cause the "murder, rape, and insanity" that the infamous Aslinger said it did. The Mafia wasn't involved, the Illuminati wasn't involved. It was racism that eventually built up to a public panic, confused parents who blamed the drug for the new rise of teenage rebellion and jazz music, and other social reasons.