The commander Haji Jawad al-Talabwi fixed me with an unblinking stare and warned me that he was a hard man.
He told me I'd better not reveal the secrets of Asaib Ahl al-Haqq - Iraq's most powerful Shia Muslim fighting group - to foreign intelligence.
We'd just heard him laying down the law on the phone, issuing a warning to whoever was on the other end that suicide vests had been smuggled in. If the Sunnis helped Isis militants they would be killed, one by one if necessary.
Haji Jawad was about 5'8" (1.72m) in his boots and combat fatigues, and somewhere in middle age. He looked like the kind of man whose threats should be taken seriously.
We were in a building they had commandeered in Djail, a small and dusty town about 40 miles (65km) north of Baghdad.
Haji Jawad is a senior military figure in Asaib Ahl al-Haqq, or League of the Righteous. He seemed to glory in his organisation's fearsome reputation. Isis, he said, knew the kind of men they were facing around Djail.
"We believe there's a divine promise that we'll win. Our enemies are filled with fear before they even see us. We've had phone calls from most of the villages held by militants, offering to surrender in exchange for their safety," he told me.
"This shows how terrified the militants are of us - it's because of our expertise in urban and guerrilla warfare, and the experience we gained from fighting against the Americans and the British."
It is not easy to find the headquarters of Asaib Ahl al-Haqq or to be admitted when you arrive.
The group, which is also an influential political movement, does not encourage casual callers. It moves its headquarters regularly.
But we had been invited to meet Sheikh Qais al-Khazali, the leader of Asaib, so there was a friendly welcome.
Fighting on two fronts
The sheikh, who's around 40, was dressed in the dark robes and white turban of a Shia cleric.
He emphasised that the group was now a political movement as well as a military force. He claimed, despite its reputation, that it was not sectarian, and would protect all Iraqis against foreign invaders.
The group emerged fighting the Americans and the British after they invaded Iraq in 2003. Among the operations that made them notorious was the kidnap of five British men in 2007, only one of whom survived to be released two years later.
The sheikh's men have also fought in the Syrian war on the side of the Assad regime, holding the area in Damascus around a revered Shia shrine.
Qais al-Khazali believes the wars in Syria and Iraq are one and the same.
"Sending our men to fight in Syria was the right decision," he said. "Al-Qaeda has had a lot of practice in street fighting. If our guys hadn't got the experience in Syria, al-Qaeda and Isis could have taken Baghdad and we wouldn't be sitting here now."
Asaib Ahl al-Haqq's current building, like so many others in Baghdad, was protected by high blast walls, razor wire and armed men.
One of the guards was wearing a tight sand coloured tee shirt. No-one seemed bothered that it was decorated with the insignia of the US Army, their old enemy, perhaps because it was one dreamt up by a fashion designer somewhere.
Inside, Sheikh Qais was blaming the West for Iraq's misfortunes.
It was America and their friends who had brought such trouble on Iraq, opening the door to al-Qaeda and its offshoots, including Isis, now the most deadly Sunni jihadist movement.
He accused Qatar of funding Isis, as part of a plan to spread "chaos" in the region. Saudi Arabia, on the other hand, was he said funding other Sunni groups, including the Nusra Front, the official al-Qaeda affiliate in Syria.
Asaib Ahl al-Haqq and its leaders make no secrets of their links with the Iranians. Their fighters are trained and supplied by Iran. They are one of the few Iraqi formations that might just scare Isis as much as Isis scares everyone else.
But Sheikh Qais said there was no need for his Iranian friends to send in their own troops.
"We don't want foreign military forces from any country. We have enough people in Iraq. We don't have to bring armies in from other countries."
"If you mean advisers, as you know now in Iraq there are not only Iranian advisors, but there are Americans. And maybe Russians for the [recently imported] Sukhoi jets."
"Iraq as a state isn't strong. It's weak. So it needs advice and weapons. But when it comes to soldiers, Iraq doesn't need anyone else to fight its wars. The Iraqi people are brave and they can defend themselves."
In the battle zone north of Baghdad his men took the BBC to positions they said were only 500m (500 yards) from Isis fighters.
The war is being fought in the baking-hot centre of Iraqi territory.
It is not just having repercussions right across the country. It is spreading instability and threat across the region, especially along the fault line that runs between Shia and Sunnis.
It might end up affecting countries further afield, in Europe and North America.
Guns dominate this country. Talk of a political deal to dilute the Shia ascendancy in politics is getting nowhere. Iraq's religious and ethnic fractures deepen by the day.