The oldest dictum of statecraft must be “the enemy of my enemy is my friend”. As the terrorists of the Islamic State (IS) advance to the very barbed wire of Turkey’s southern frontier with Syria, this telling phrase helps explain recent events.After a lightning offensive across Syria, IS is now fighting within 100 yards of Turkish territory. Never forget that the stated ambition of Abu Bakr al-Baghdadi, the movement’s leader, is to “conquer Rome” by way of Istanbul.Given that Baghdadi has managed to deploy tanks within sight of the barren hills of southern Turkey, you might have thought that Recep Tayyip Erdogan, the country’s new president, would want to use all necessary means to drive IS away from the border. After all, Turkey has the second bi