Johann Hari's latest book, Chasing the Scream, chronicles the war on drugs in the United States, beginning with the passage of the anti-narcotics Harrison Act in 1914 and running up to near present day. It's one of the best books I've ever read, rich with page after page of myth busting science, social research, and history showing how basically everything we think we know about drugs and addiction is wrong.

There's loads of useful information in the book, but there's one particular section I especially cannot get out of my head. I've never heard an explanation for the drug war's racist disparate impact that makes so much sense. Each day as I read the news about new surveillance technologies deployed primarily against black and Latino people, and look around at our mass incarceration society, the following anecdote fills in the gaps.

Hari, like many of us, wanted to understand why US policing in the drug war is so fundamentally racist. So he went looking for answers. Here's what one police officer told him:

Matthew Fogg is one of the most decorated police officers in the United States, responsible for tracking down more than three hundred of the most-wanted felons in the country—from murderers to rapists to child molesters. But he was bewildered as to why his force only ever goes to black neighborhoods to bust people for drugs. He went to see his boss to suggest they start mounting similar raids in white neighborhoods.

He explained in a speech that his superior officer told him: “Fogg, you know you’re right they are using drugs there [but] you know what? If we go out and we start targeting those individuals, they know judges, they know lawyers, they know politicians, they know all of the big folks in government. If we start targeting them, and their children, you know what’s going to happen? We’re going to get a phone call and they’re going to shut us down. You know that, Fogg? You know what’s going to happen? There goes your overtime. There’s the money that you’re making. So let’s just go after the weakest link. Let’s go after those who can’t afford the attorneys, those who we can lock up.”

Whites use drugs at about the same rate as blacks. Some studies have found that whites are more likely to sell drugs than their black neighbors. But whites are many times less likely to be arrested on drug charges, for selling or for possession, than black people. Historical racism starting with slavery, and the creation of modern police forces as tools to control black people in the post-reconstruction era certainly play a major role.

Some people explain away this historical (and present) racism by suggesting that this disparity is a result of the different drug markets in white and black communities. White people sell drugs inside to their friends and families, and black people sell them outdoors to strangers, that argument goes.

But that can't be it. The explanation provided by the cop quoted in Hari's book seems like the missing piece of the puzzle. Put quite simply: If police treated white people the way they treat black people in the drug war, this travesty of justice we call prohibition wouldn't have lasted a decade.