The Wall Street Journal is reporting that Iran sent two battalions of Iranian Revolutionary Guards to help the Iraqi government in its battle against the Islamic State in Iraq and the Levant (ISIS). Not to put too fine a point on it, but this is hugely important, if not totally surprising given Iran's intervention in Syria. Iran has the power to crush ISIS in open combat. But Iranian intervention could also make the conflict inside Iraq much worse.

combined Iranian-Iraqi forces have already retaken about 85 percent of Tikrit

These aren't just any old Iranian troops. They're Quds Force, the Guards' elite special operations group. The Quds Force is one of the most effective military forces in the Middle East, a far cry from the undisciplined and disorganized Iraqi forces that fled from a much smaller ISIS force in Mosul. One former CIA officer called Quds Force commander Qassem Suleimani "the single most powerful operative in the Middle East today." Suleimani, the Journal reports, is currently helping the Iraqi government "manage the crisis" in Baghdad.

According to the Journal, combined Iranian-Iraqi forces have already retaken about 85 percent of Tikrit, a city in north-central Iraq and Saddam Hussein's birthplace. That alone demonstrates the military significance of Iranian intervention: Iraqi forces have previously floundered in block-to-block city battles with ISIS.

a strong Sunni support base is the key to ISIS building up a powerful enough force to maintain an effective rebellion

But these sorts of victories could prove ephemeral quickly, as Shia Iran's intervention could infuriate the Sunni Muslims whose allegiance ISIS needs to win in the long run. The internal Iraqi conflict is firmly sectarian: ISIS is a Sunni Islamist group, and the Iraqi government is Shia-run (a majority of Iraqis are Shia). Iran is also a Shia state.

Iranian intervention in the conflict could convince Sunni Iraqis who don't currently support ISIS to shift their allegiances. The perception that the Iraqi government is far too close to Iran is already a significant grievance among Sunnis. That's part pure sectarianism and part nationalism. Many Iraqis don't like the idea of a foreign power manipulating their government, particularly Iran (memories of the Iran-Iraq war haven't faded).

Iranian participation in actual combat risks legitimizing ISIS' propaganda line: this isn't a conflict between the central Iraqi government and Islamist rebels, but rather a war between Sunnis and Shias.

That risk becomes much higher if joint Iraqi and Iranian units kill Sunni civilians during the fight against ISIS — which the Iraqi military has done in the past. Indeed, the Iraqi government's brutal repression of Sunnis is one of the core reasons ISIS managed to recruit enough troops to challenge the Iraqi government in the first place. And a strong Sunni support base is the key to ISIS building up a powerful enough force to maintain an effective rebellion, as Brookings Doha's Charles Lister explains:

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Iranian intervention in the Syrian conflict has helped Syrian dictator Bashar al-Assad hold on to power when it looked like all was lost for him. We'll see if it ends up similarly helping the Iraqi government — or undermining it.