In 1994, John Ehrlichman, the Watergate co-conspirator, unlocked for me one of the great mysteries of modern American history: How did the United States entangle itself in a policy of drug prohibition that has yielded so much misery and so few good results? Americans have been criminalizing psychoactive substances since San Francisco’s anti-opium law of 1875, but it was Ehrlichman’s boss, Richard Nixon, who declared the first “war on drugs” and set the country on the wildly punitive and counterproductive path it still pursues. I’d tracked Ehrlichman, who had been Nixon’s domestic-policy adviser, to an engineering firm in Atlanta, where he was working on minority recruitment. I barely recognized him. He was much heavier than he’d been at the time of the Watergate scandal two decades earlier, and he wore a mountain-man beard that extended to the middle of his chest.