The war on drugs was the first major test by the country's elite to see if Americans would willingly surrender their constitutional rights. It turned out that they would and so for the past twenty years invasions of civil liberties increased, America threw more and more of its young people into prison, while exploding drug war budgets did nothing to stem the growth of the drug industry. Further, the drug war was a useful testing ground for repressive measures instituted following September 11.
But to make all of this work you need a sufficient quantity of drugs, they had to be easy to find and a sufficient number of people had to use them. This is where marijuana came in. Although marijuana is far less danger than just legal drugs as cigarettes and alcohol and, even as a medical prescription, far less hazardous than ones routinely given out by doctors, it had the constituency, physical bulk and ubiquity to make it just the thing for adding to police budgets and taking away from human rights.
The war on drugs will undoubtedly be regarded by historians as a crucial precursor of the end of the First American Republic. It tested the waters of repression and found Americans willing to accept it. Even liberals outside of strong civil liberties advocates proved disastrously indifferent to what was going on.