Marijuana was made illegal in the early 20th century because US policy makers (and other governments around the world, but mostly the US) believed that wide spread use of the drug amongst ethnic underclasses would be a catalyst of revolution against the government and the start of the civil rights movement (which only happened later anyway, cos y'know, you cant keep a brother down).

I will now risk undermining my whole argument by straying into conspiracy theory:
Most illegal drugs are nowhere near as harmful to your health as tobacco or alcohol (fact), but because most of them are stimulants, as opposed to a depressant, (like alcohol) (again, fact), they are made illegal because our governments and the corporations that run them (conspiracy) are fearful that in using said drugs, we will all figure out that they are shitting on us from a very great height, and thus will demand change instead of just wiping the shit off and carrying on being their depressed-as-fuck slaves (conspiracy).

If alcohol and tobacco were introduced today as brand new substances, they would go straight to Class A drugs - they are both highly addictive, highly destructive to your health, and claim many millions of lives per year.

*disclaimer: the use of comic-sans in the above video in no way undermines it's validity

Much love,

Craigus