In 2014, at its apex, the Islamic State group (IS) put out slogans about reaching Rome. It published doctored pictures showing its black flag raised over the Vatican, denoting its stated aspiration to conquer one of the most important symbols and power centers of Christianity in the world.

Seven years later, Pope Francis has just visited Mosul, the city where former IS leader Abu Bakr Al-Baghdadi delivered his famous sermon declaring the rise of the "Caliphate" between Iraq and Syria. The Pope led prayers amid the ruins...