history

Prohibition was the prototype for the nationalization of law enforcement in the US, and thus the literal creation of the standing army we've had in this country since 1919. It became so 'useful' for the feds to have that army, that when Prohibition was repealed, a substitute "reason" was urgently 'needed'.

There was no way somebody like FDR was going to let a federalized law-enforcement agency go in the middle of a Depression, when the Bonus March military crackdown and rumblings of a 'business plot to overthrow the government' were fresh in everyone's minds.

"Narcotics" became a 'thing' in 1934. Why then? To prevent that nationwide suppression force of former alcohol Prohibition-enforcers from becoming unemployed by repeal of the 18th Amendment.

The Propaganda Division Hollywood chimed in with "Reefer Madness" and other drug hysteria movies, some with literal government co-production.

So there we have it; completely set up before WWII. Just add a little bureaucratic inertia, boosters from Nixon, Reagan, Clinton, and Sessions, and here we are! And here we're going to stay, regardless of which puppet sits in the WH on Jan 21st.