Cannabis prohibition has an ugly, racist history (which you’ll know if you read my other posts about the history of cannabis). But don’t just take my word for it. Check out this lawyer-made infographic.
I have no affiliation with the author and her data ends in 2013 so might not reflect current state laws, but it’s good information. I did notice one error — for 1996, California made cannabis legal in the state (not the country ;-)) — but if you’re visual, this may help you understand a bit more. Click the image for the full-sized version.