Masked Shi'ite fighters hold their weapons in Al Hadidiya, south of Tikrit, en route to the Islamic State-controlled al-Alam town, where they are prepared to launch an offensive, March 6, 2015. Thaier Al-Sudani/Reuters The "most influential commander" of the Shia militias that have become crucial in the fight against the terrorist group ISIS is a US-designated terrorist himself and has close ties to Iran, according to a new report from The Wall Street Journal.

To Shia militia fighters in Iraq, Jamal Jaafar Ibrahimi is "singular leader, arranging logistics and military plans" for much of the Popular Mobilization Units (PMUs) and the right-hand man of perhaps the most notorious Iranian commander in the Middle East — Qassem Soleimani.

Ibrahimi reportedly carries out the operations Soleimani plans for Iraq. And his influence extends beyond just the Shia PMUs — he's known to have "de facto control of other forces, including army and police, on parts of the battlefield where the militias lead the fight," according to The Journal.

Iranian-backed PMUs once targeted US soldiers during the Iraq war. But they're now Iraq's "strongest combat force," and the US leans on them for ground operations in the ongoing fight to defeat ISIS (also known as the Islamic State, ISIL, or Daesh).

The Washington Post recently examined the threat from the Iran-backed Shia militias that have become increasingly important on the battlefield, most recently in Fallujah, an Iraqi city that forces are fighting to liberate from ISIS.

Hugh Naylor and Mustafa Salim explained:

These government-aligned militiamen have helped push the Islamic State out of key areas of the country but also have become a complication for the US-backed military coalition assembled to destroy the hard-line Sunni group. They filled an important void left by Iraq's weakened armed forces, but their religiously motivated agenda has aggravated Iraq’s combustible sectarian divisions.

... The militias ... have a reputation for brutal reprisals against Sunnis suspected of being loyal to the Islamic State. Iraqis and human rights groups have accused them of torture, forced disappearances and executions.

The Journal's story highlights the difficult choice between allowing Iranian influence in the region, which could bring even more sectarian division along with it or increasing US military involvement to defeat the terrorist groups the militias are currently targeting in the Middle East.

So far, it looks like Iran is winning.

Zalmay Khalilzad, a former US ambassador to Baghdad, told The Journal that Iran's militias are "the infrastructure of future civil wars."

Although US officials have been warning against Iranian influence in Iraq, they haven't been able to stop it. Despite Ibrahimi's terrorist status, he has a residence near the US embassy in Baghdad's Green Zone.

"He's a designated terrorist," Col. Steve Warren, a spokesman for the US-led anti-ISIS coalition, told The Journal. "That's how we think about him. But he's there, so he's got to be accounted for."