WESTERN LEADERS sometimes suggest that the Islamic State is its own worst enemy, so extreme in doctrine and practice that it will galvanize opposition within the Islamic world. While that is proving true to some extent — Muslim governments, senior clerics and even other jihadist groups have joined the fight against the would-be caliphate — the sobering truth is that the Islamic State also has picked up popular support and the allegiance of other militants in countries as far away as Algeria and Pakistan.