With a harsh dispute between Iraq’s Defense Ministry and the Pentagon over the Mosul offensive, the Iraqi military decided to go with an invasion of Tikrit without any US involvement, and without any consultation with the US.

It’s one of the biggest offensives yet against ISIS-held territory, but Iraq isn’t going it alone. Rather, it seems, Iran is taking a key role in the offensive, with artillery on the ground and Shi’ite militias that are close to the Iranian government leading the charge.

The key militia in this case is the Badr Brigade, formerly led by Iraq’s ex-PM Nouri al-Maliki, and which retains huge influence in the Iraqi government to this day. Historically, the Badr Brigade has been tightly allied with Iran as well.

And so, as this major offensive begins, the US is on the outside looking in, leaving the Pentagon complaining that Iran is usurping their role as leader of the anti-ISIS war. Given the acrimony over the Mosul fiasco in recent weeks, if the Tikrit offensive goes well Iraq may find Iran a preferable ally.