Islamist extremist group has capitalised on instability to control a swath of the region

The scene is wearily familiar. It is dusk at a ramshackle military outpost, surrounded by miles of scrubby desert or on the outskirts of a major town.

Suddenly, there is the sound of automatic rifle fire, and hundreds of men arriving on motorbikes, then explosions, screams, fire, smoke. The defenders flee or are killed. The attackers shout triumphant cries of “God is Greatest”.