Notorious Islamic State ‘Beatles’ in U.S. custody, but can 11,000 other detainees be held securely?

The ragged band of Western hostages, enduring captivity in the heart of Islamic State’s self-proclaimed caliphate, somehow summoned the grim humor to nickname the four guards with distinctive English accents who tormented them daily.

They called them “the Beatles.”

Eventually, three Americans among the captives held by the cell in 2014 and 2015, together with several other foreign hostages, would be beheaded in the bleak Syrian desert, with the killings recorded in grotesque...