The Iranian regime knows it lacks the military power and the political support needed to seize direct control in any Arab state, least of all Iraq. This is why it plans to create a state-within-a-state situation — where the formal government in Baghdad, like the formal governments in Beirut or Damascus, will be an empty shell, with real power exercised by heavily armed and well-funded groups linked to Tehran. These Iranian-controlled groups would command chunks of territory while letting Sunni jihadists set up shop in their own neck of the woods. In other words, Iran is not aiming to defeat IS, let alone destroy it. All Tehran wants is to create a safe corridor through Iraqi territory to Syria and thence to Lebanon.

We're letting Iran and ISIS carve up Iraq, New York Post (March 15, 2015).