The first emails from Iraq landed in John Kael Weston’s inbox while he was eating breakfast at a Utah ski resort. Islamic Statefighters had just seized Fallujah, and the former State Department diplomat fired off a worried message to the Iraqi policeman who helped him over and over again during the war’s darkest days.

“Are you and your father and family ok?” Mr. Weston asked Saad Abu Fahad. The American and Iraqi were neighbors in Fallujah, taught each other about their country’s politics and always stayed in touch by email....