The war on drugs has it's roots in racism. For a long time the government didn't make any of them illegal, but the first attacks were against opium being shipped in by immigrant Chinese. They played around with various levels of tarriffs for a while, in an attempt to stop it.

I believe that marijuana was the first drug made illegal, and done so because it's use was mainly among non-whites, though it was used in a large variety of forms by many people.

Nowadays, them claim it's for the benefit of people, though somehow tobacco, more addictive than almost any other drug, and causing more deaths than any other drug, is not only legal but subsidized by the government.

There is some suspicion that the war on drugs is kept alive for the wealth it brings the government, due to horrific search and seizure laws enacted to "combat" drugs. There is so much latitude that the government systematically ruins peoples' lives if there's even a possible connection to drug money, by taking all the property. I've even heard of a case where a woman's house was seized because her son sold drugs from there once.