Imagine if a fundamentalist Christian sect captured the French city of Lyon and began a systematic purge of Muslims. Their mosques were destroyed, their crescents defaced, the Koran burned and then all Muslims forced to flee or face execution. Such an event would be unthinkable today, and if it did occur Pope Francis and all other Christian leaders would denounce it and support efforts by governments to stop it.

Yet that is essentially what is happening in reverse now in Mosul, as the Islamic State of Iraq and al-Sham drives...