Here in the US, most of us think of Prohibition in exactly those terms.... but that didn't stop us from doing the exact same thing all over again and calling it good.
(Watching Ken Burns "Prohibition" is something I'd strongly recommend to just about anyone, not only is it a fantastic documentary as par for the course for Burns, but it was a steady stream of "HISTORY REPEATS ITSELF" in such stark terms as you rarely see.)
As other posters have suggested (and been duly modded down for it), prohibition isn't really about the drugs.
http://www.pbs.org/kenburns/prohibition/
Before I watched it I thought "how could people be so stupid as to think they would ever ban alcohol". After I watched it I was like "oh wow, people drank on average 1.5 bottles of hard alcohol a week? Drunks were everywhere? Opening a bar was super cheap because the alcohol companies subsidised them. Ok, maybe I get why people would vote for prohibition".
I'm not saying it was a good idea. It was clearly stupid idea which the rest of the doco made clear and is directly analogous to the war on drugs.
Another POV that gave me a new perspective was the idea that legal drugs (alcohol) kill way more people than the war on drugs kills. 3+ million a year die from alcohol. (http://www.who.int/substance_abuse/facts/alcohol/en/). Legalizing a bunch more drugs may have the unintended consequences of killing an order of magnitude more people than the war on drugs.
I'm not saying I'm for the war on drugs. All the unintended consequences of that are horrible as well. The USA's police state, seizure laws, militarization, etc. But, the idea that we'll end up possibly causing 10x more death from legalization makes me wonder if there's some middle position
While I'm not necessarily all in favor of weed, for example, I'm convinced that legalizing it, simply because of its properties, will not lead to even a fraction of the deaths that legalized alcohol use does.
It's difficult to discuss any kind of position when we use a term as vague as 'drugs'.
Trying to foresee all future consequences is pointless when you have no control over them anyway. Besides, how many lives have been destroyed by the War on Drugs? That requires second and third order reasoning, not simply a body count.