After serving his time federal prison, John Ehrlichman granted an interview to author Dan Baum, who reports that Ehrlichman explained the origin of the war on drugs this way:
The Nixon campaign in 1968, and the Nixon White House after that, had two enemies: the antiwar Left, and black people. You understand what I’m saying? We knew we couldn’t make it illegal to be either against the war or black. But by getting the public to associate the hippies with marijuana and blacks with heroin, and then criminalizing both heavily, we could disrupt those communities. We could arrest their leaders, raid their homes, break up their meetings, and vilify them night after night on the evening news. Did we know we were lying about the drugs? Of course we did.
Source: The Moment: Wild, Poignant, Life-changing Stories. Edited by Larry Smith, Harper Perennial, 2012.