The Marijuana Conviction by Richard J. Bonnie and Charles H. Whitebread II
The only reason marijuana is illegal is that it was introduced to the country by non-European ethnic minorities, during a time when the country was dealing with widespread opiate and cocaine addiction, and because Prohibition and the social forces surrounding it had firmly introduced the 'illegalization' model in dealing with drug use into the country's psyche.
(I take it that no one any longer wrongly believes marijuana to pose an actual public health risk - if you do, kindly hop in a time machine to the thirties where you belong.)
Due to fear of 'the other', non-European ethnic minorities and their drug, and the profit motive applied unscrupulously to the newspaper business, an anti-drug crusader at the head of the then-Federal Bureau of Narcotics was convinced that marijuana was an incredibly dangerous substance (it led Negroes and Mexicans to rape white women, for instance). He in turn added to the anti-marijuana propaganda blitz, which through lurid, misleading anecdotes and spurious "scientific" research which ignored the body of scientific research dating from before the propaganda blitz, convinced the public that marijuana caused insanity and inexorably led to violence and sexual crimes. His successful propaganda efforts got Congress to write laws sending users of a pharmacologically harmless substance to federal and state rape, labor and human rights-denying camps - prisons - for many years.
Once middle and upper class white kids started to use it in the 60s - woah, then things got to changing. "The scientific propositions attending the application of the narcotics consensus to marihuana had always been assumptions tied to broader social perceptions of the using class. But these assumptions no longer coincided with social expectations when use of the drug was taken up by society's privileged classes. The basic proposition that use inevitably became abuse was quickly challenged. ... Similarly the causal relationships between marijuana and crime, idleness, and incapacitation were now more difficult to maintain. The new users were not 'criminals' or social outcasts. They were sons and daughters of the middle and upper classes. In short, when the consensus against marihuana lost its sociological support, it immediately lost its scientific support as well."
In other words: poor black and brown people being sent off to rape camps for doing nothing harmful? That's OK. But when Billy and Susy start getting kicked out of university for a five year prison sentence... well let's take a look at that marijuana science again. The 60s' and 70s' saw a renaissance of actual scientific investigation into marijuana, after a long hiatus, and found that marijuana is a remarkably nontoxic substance. Some tried to translate science into policy, but with limited success.
However, the public propaganda starting in earnest in the 30s had been too successful. Even now, "cultural conservatives" ensure that a rational marijuana policy is simply too much for the United States to implement.
The upside of this policy of sending users of a harmless substance to mental health-destroying federal and state rape and institutionalized violence camps? Once people realize that marijuana is harmless and its criminalization is the summit of both cruelty and absurdity, then they are, with respect to government policy, down the rabbit hole. Which in the United States is very, very deep.