Four years ago, I wrote a letter to John Walker Lindh, then inmate number 45426-083 in Terre Haute penitentiary, to ask for advice about jihadism, Islamic law, and the Islamic State. Lindh is the most famous jihadist America has ever produced. In December 2001, he was pulled, half-dead, from a cellar full of fellow Al Qaida fighters in northern Afghanistan, and 10 months later he was sentenced to 20 years in U.S. prison for terror-related crimes. He is scheduled to be freed today, with three years off for good behavior, and many—including Donald Trump—have objected to his release.

The story of our correspondence is in my book, The Way of the Strangers: Encounters with the Islamic State, but some details appear here for the first time.