Across large areas of Iraq and Syria, an ideologically radical religious army, Isis, has declared itself a relentless foe of modern civilisation, seeking violently to replace the international system’s multiplicity of states with a single Islamic empire governed by Sharia law. In these circumstances, the traditional adage that the enemy of your enemy can be regarded as your friend no longer applies. In the contemporary Middle East, the enemy of your enemy may also be your enemy. The Middle East affects the world by the volatility of its ideologies as much as by its specific actions.

The outside world’s war with Isis can serve as an illustration. Most non-Isis powers -- including Shia Iran and the leading Sunni states -- agree on the need to destroy it. But which entity is supposed to inherit its territory? A coalition of Sunnis? Or a sphere of influence dominated by Iran? The answer is elusive because Russia and the Nato countries support opposing factions. If the Isis territory is occupied by Iran’s Revolutionary Guards or Shia forces trained and directed by it, the result could be a territorial belt reaching from Tehran to Beirut, which could mark the emergence of an Iranian radical empire.