After the American pull-out, Iraqi Shi’ite sectarianism undermined the army of Iraq as a fighting force. In Summer 2014, the fundamentalist Sunni movement called the Islamic State (ISIS) attacked out of eastern Syria into Iraq. The army of Iraq collapsed. Then Iraqi Shi’ite sectarianism came to the rescue. Various Shi’ite militias, under the umbrella term Popular Mobilization Forces, were called upon to save the day. At first blush, the militias didn’t seem capable of stopping the advance of ISIS forces, which quickly over-ran Mosul and drew close to Baghdad itself.
However, the Sunni-Shi’ite civil war has long involved Shi’ite Iran’s support for embattled Shi’ite regimes elsewhere. Iran has been a principal ally of Bashar al Assad’s regime in Syria. When ISIS tore into Iraq, Iran responded with aid and support. The Iraqi militias received a lot of training, weapons, and direction from Iran’s Revolutionary Guard Corps. To whom are the militias more loyal, the government of Iraq or the government of Iran? The issue is going to become pressing as the Assad regime appears to be triumphing in western Syria and the Iraqi offensive erodes the ISIS position in western Iraq.
Having sectarian militia featured too prominently in the fight against ISIS posed all sorts of problems for the government of Iraq. So, the brunt of the attack on ISIS-held Mosul has been born by the regular army of Iraq, backed-up by paramilitary national police. The Shi’ite militias were deployed to cut off the lines of communication between Mosul and the ISIS stronghold in eastern Syria. In the course of their operations, the militias have seized some of the territory along the border with Syria.
Once Mosul falls, will the Iraqi militias cross the border into Syria? Iraqi militia intervention in Syria would raise another set of problems. The Assad regime is beset by ISIS, but also by Sunni “moderates” and by Syrian Kurds. Could/would the Iraqis limit themselves to fighting ISIS? The Assad regime, backed by Russia and Iran, has concentrated its attacks on the Syrian “moderates.” Would the irruption of a foreign Shi’ite military force into Syria reignite Sunni resistance? Shi’ite fighters from Lebanon’s Hezbollah have long been an important prop to the Assad regime. More recently, Iraqi Shi’ite militia troops were airlifted in to western Syria to join the fight against the “moderate” Sunnis.
Turkey, which has reduced its opposition to the Assad regime while continuing to support rebel forces that are fighting against ISIS, is deeply alarmed by the advance of Kurdish forces in Syria and Iraq because this threatens to create a Kurdish proto-state outside its borders. That Kurdish state could support Kurds within Turkey. The United States has long insisted that the fight against ISIS should be the real point of concentration for military efforts in Syria.
Spokesmen for the Syrian “moderate” rebels are insisting to Western journalists that Iraqi intervention in eastern Syria would be a disaster. Iraqi intervention “will cause a sectarian ignition,” said one. It “will ruin everything” said another, perhaps causing Sunnis to flock to ISIS as their only defense against the Shi’ites. More significantly, president-elect Donald Trump has opposed further Iranian expansion. Similarly, Saudi Arabia would be even more alarmed—if that’s possible–by Iranian proxies intervening in force against Sunnis on a new front.
The decision on this question may rest with other people. Both sides in the Syrian civil war are close to or past the point of exhaustion. Vladimir Putin, ruler of the regional power, has chosen not to worsen relations with the United States by taking up President Obama’s challenge. He may prefer to insist on a pause before any action in eastern Syria.
 Yaroslav Trofimov, “Will Iraq’s Shi’ite Militias Cross Into Syria Next,” WSJ, 30 December 2016.