Islamic State’s rout in the northeastern Iraqi town of Hawija last week heralded the jihadi group’s demise as an ideological, political, military and economic force in Iraq and Syria.

Islamic State, also known as Isis, seemed unstoppable when three years ago its fighters swept from Syria into northern Iraq, conquering Mosul and large swathes of territory.

Islamic State’s dramatic succession of military victories attracted thousands. The Britain-based Syrian Observatory for Human Rights estimated it had 80-100,000 fighters while the authoritative The Soufan Group said the number of foreign fighters had risen from 12,000 to 31,000 in 18 months…