A pretty well sourced and footnoted account of marijuana prohibition can be found on the drugwar rant site.
The article contains some eye-opening quotes, for example, from a 1934 newspaper editorial:
“Marihuana influences Negroes to look at white people in the eye, step on white men’s shadows and look at a white woman twice.”After all, you couldn't have a black look a white in the eye, could you.
The American Medical Association opposed the new Bill because there was no evidence whatsoever to support the lurid claims of prohibitionists. Then, as now, such sober expert advice was ignored.
There is also an interesting re-telling of the origins of the term 'assassin', a word derived from 'hashashin'. The popular story goes that an ancient eastern cult of hired assassins would get high on hash to drive their murderous brutality. This was used in the 1930s as evidence that marijuana made people violent, Reefer Madness style.
But according to Drug War Rant author Peter Guither, the real story was that the ruler would invite the assassins to the palace gardens and get them stoned to create a vision of the paradise that awaited them when they had fulfilled their political mission. Then, sober, they would go kill people.
Prohibitionists, however, would say all the above was then, this is now, and lots of new evidence has been found about the harms of cannabis. What harms have been found from $billions poured into such research, however, are minuscule compared to the harms of most activities or are 'linked' to only a tiny, tiny percentage of users.
But much of the 'evidence' is still biased rubbish, even today. For example NCPIC, our government-funded anti-cannabis agency, published a 'study' attempting to link cannabis use with violence. Nothing changes.
If it leads to violence, one wonders why the 250,000 stoners at Woodstock didn't all kill each other instead of dancing in the rain.
Regarding governments ignoring the experts, see the recent sacking of Prof David Nutt in the UK for questioning the government's politics-driven re-criminalisation of cannabis against the advice of its peak drugs advisory body.