1840 China. Opium was legalized. 60 years later, Half the population of Manchuria was addicted.
Their three thousand year old form of government collapsed a decade later. The nation went through one rack and ruin after another until a Dictator, taking advantage of all the social upheaval, finally ended up shooting all the drug addicts and ruling thereafter with an Iron fist for the next 40 years. 
70 million people died, and it all started with the societal collapse caused by legalized opium.