A great deal of the credit/blame for the modern drug war goes to Harry Anslinger[0]. Imagine being the nation's top enforcer of Prohibition - what do you do when it's repealed? Just close up shop, fire everyone, and turn the lights off before you lock the door?

This is a somewhat partisan issue and I'm far from unbiased, but reading his wiki page will convey most of the story better than a screed.

0: https://en.wikipedia.org/wiki/Harry_J._Anslinger