I'd been communicating with Russell Dennison almost daily for more than six months when his messages suddenly stopped coming.

Dennison was a devout Muslim. He believed the time and place of his death were predetermined, that if a bomb were meant for him, it would kill him, regardless of anything he might do to avoid it. That bomb found him in the spring of 2019 in Baghuz, a small village in eastern Syria near the border with Iraq. I learned of his death months later, after a witness told Dennison’s Syrian...