Through the bars of his prison window Rédoine Faïd can see far off into the cloudless sky. It's early on a sunny Sunday in July 2018, and for now, the morning is quiet and ordinary at the Réau penitentiary, 25 miles southeast of Paris. But Faïd can envision what's coming—he can see it all unfold like the movie he's been scripting for months in his mind. Outside his cell, two guards approach. These are the solitary confinement quarters: a controlled unit within the maximum-security prison where notable or potentially dangerous criminals are held. Few prisoners in France are as notable as...