BAGHDAD - The civil war is between the two main branches of Islam -- the Sunni and the Shiite. Prime Minister Nouri al-Maliki's government is predominantly Shiite. The attacking force, ISIS, is Sunni.
ISIS now holds parts of northern Syria and western Iraq. In recent days, three border crossings along Syria and U.S. ally Jordan fell to ISIS, which is using extreme violence to create a new nation with a strict Islamic code.
By now it has become a familiar sight: ISIS militants parading through newly captured territory, this time the border town of al Qaim.
Iraqi forces have lost control of all of the main border crossings into Syria, as well as four towns along the Euphrates River.
With the Iraqi army in disarray, Shiite militias are stepping in. They are heavily armed and highly motivated, and many fear they will heighten sectarian tensions.
We traveled to the conservative, Shiite enclave of Sadr City, home to the Mehdi Army, one of the most powerful militias in Iraq.
American soldiers fought some of their toughest battles here. But on Monday, tribal leader Abu Ahmed Lafraiji told us the U.S. is an ally in the fight against ISIS.
"We will pay any price, people will die, blood will be spilled," he said. "The most important thing is to kick them out of Iraq."
Do the Shiite militias want help from America?
"I swear we only need their planes," he said. "We don't need their army because we have our own. But we need help with weapons, vehicles and planes."
Like tens of thousands here, two of Abu Ahmed's sons have volunteered to fight in Samarra, site of one of the holiest Shiite shrines.
We asked him why they went.
"To defend the country, of course," he said. "After the grand cleric called for jihad, all of us, not just our children, we must all go to fight jihad."
When you talk to officials, particularly when you speak to them privately, it's striking just how pessimistic they are. There's a real sense that the Iraqi army simply cannot win this on the battlefield without major international support.
And that means all hopes are pinned on Iraqi leaders quickly being able to form a national unity government, which frankly would be a real departure from past experience here.