It's also an eye-opener in revealing the historical origins of the expansion of police powers like wiretapping, and IIRC, overreaches of the Interstate Commerce Clause.
Once you've watched it, it's easy to draw some very damning "learn from history of be doomed to repeat it" parallels between Prohibition and the War on Drugs at a more detailed level than "drug prohibition doesn't work, just look at Prohibition."
There's a healthy bit of conspiracy theory that says the real motivation for the marijuana prohibition was a Dupont family political move against Henry Ford, who was looking at alternative fuels based on hemp. Certainly, Harry Anslinger, the primary campaigner for anti-marijuana laws, was a Dupont in-law, lending credence to the theory. Still, it's conspiracy theory and should be taken with a grain of salt.
But in the longer political run, marijuana makes an excellent nuisance crime for harassing social undesirables - blacks, latinos, and white hippies and artists who were unduly influenced by black culture. That's not a matter of research, it's just plain obvious. And since it's kind of hard to actually show that marijuana causes black men to rape white women or that it leads directly to heroin and communism or whatever, there is quite a cottage industry in faux-science to "prove" the dangers of a basically harmless herb.
The bigger problem in modern years, imho, has been the rise of the private drug testing industry. It doesn't hurt the self-image of big businesses to demand that potential employees pee in a cup to prove their competence, but it's completely shameful socially. It's ridiculous.
the KKK started out as an Anti catholic group
The 'second' Klan was originally a merchandizing effort surrounding the movie "Birth of a Nation." It quickly turned into an actual political organization which adopted the white nativist views which were in vogue at the time. It was largely anti-immigrant focusing on Catholics, but also targeting Jews and Eastern Europeans.
It's worth noting that in all of it's incarnations, the Klan was racist and anti-black. It wasn't until the 1960s that this became the center-point of their politics, but it was always there.
It's on Netflix
Here's a quick review from the Guardian to get you a taste: http://www.youtube.com/watch?v=ryCfjX7DRqM