Europe has resisted taking back citizens who joined ISIS. Now, it may not have a choice.

By and Souad Mekhennet,

Fabrizio Bensch Reuters

BERLIN — Bint Dahlia was 33 when she left Germany with her husband and children to start life in the Islamic State’s newly declared caliphate.

She is one of thousands of Europeans who did so — and, five years later, one of hundreds trying to come...