Iran may view its involvement as a necessary move to preserve what has been a hugely important strategic ally in the region, which has provided Iran with a conduit for its military support of another regional dictator, Bashar al-Assad. The loss of either or both of these allies could be Iran’s biggest setback in 25 years, according to former commander of US Central Commad, General James Mattis, as paraphrased in Business Insider.
Iranian involvement in Iraq is already beginning to look quite similar to its involvement in Syria, where it has successfully prolonged Assad’s hold on power, with many analysts suggesting that he would have been unseated by the opposition if not for Iranian arms and fighters. Yet Iran has largely denied active participation in the Syrian conflict, sometimes explaining the deaths of Revolutionary Guards there by saying that they were merely defending Shiite shrines.
Iran preemptively used this cover for its actions in Iraq, according to the Daily Beast, which quotes the Iranian chief of police and an Iranian diplomat as saying that they will do whatever it takes to defend the Iran-Iraq border and Shiite holy sites against the opposition fighters in Iraq.
But the article also points to the extent of distrust of the Iranian regime, and quotes a member of the Syrian opposition as accusing Iran of arming an extremist group in Syrian in order to try to manipulate public sentiment, even though it plans to fight this very same group in Iraq. Both actions would theoretically help to secure Iranian influence in each country over the long term.