Ahmed Saad/Reuters
"Once a leading killer of American troops, the militia is spearheading the fight against the Sunni extremists of the Islamic State, also known by the acronyms ISIS and ISIL. That means Asaib Ahl al-Haq and the United States military are now fighting on the same side, though each insists they will not work together," writes David D. Kirkpatrick of The New York Times.
Shiite militias, responsible for extra-judicial killings of Sunni civilians throughout Baghdad and southern Iraq, pose the largest hurdle to any US intervention against ISIS in the country.
These Shiite militias, which have swelled in size and prestige since ISIS's blitz through Iraq in June, often function as direct proxies of Iran. The largest of these militias in Baghdad is Asaib Ahl al-Haq. The militia might have a common foe in ISIS, but it's no friend to the US.
The Shiite militias have swelled with recruits and supplies, and may have reached the point of being more effective and better-supplied than the Iraqi Security Forces.
However, the militias have a history of targeting American soldiers throughout the US occupation of Iraq. The groups are also feared for their savagery when targeting Sunnis.
Images reportedly depicting Asaib Ahl al-Haq militiamen posing with charred corpses of Sunnis circulated online the same day that ISIS released the video of American journalist Steven Sotloff's execution.
Between June 1 and July 9, Human Rights Watch documented the extra-judicial execution of 61 Sunni men by various Shiite militias in the regions around Baghdad. The pace of violence is increasing: at the end of August, Shiite militiamen gunned down 68 Sunnis in a single attack on a mosque north of the capitol.
Various Shiite militias have pledged to target Americans should soldiers be placed on the ground. Kurds in Baghdad have also reported being detained and tortured by Shiite militiamen, further undermining attempts at building an inclusive government in Baghdad.
Even if the US and the Shiite militias settle into a marriage of convenience against ISIS, the joining of US and Shiite forces could be a propaganda boon for ISIS.
ISIS is largely able to function throughout the Sunni regions of Iraq due to deep dissatisfaction among Iraq's Sunni minority against what it sees as intense discrimination by the Shiite majority government.
Collusion between the US and paramilitary Shiite groups would further alienate Sunnis, lending strength and support to ISIS. For the US to avoid deepening sectarianism in Iraq, it must proceed with extreme caution so as not to appear to be supporting Shiite forces and Iran at the expense of the country's Sunnis.
ISIS is also currently locked in a civil war against the regime of Syria's Bashar al-Assad, which is backed by Iran. ISIS can use any military action by the US that is seen as helping Assad in order to demonstrate America's deepening involvement in what the group sees as a region-wide Sunni-Shiite war.