To the Kurds of Iraq's north, the sight of their vaunted peshmerga troops in full retreat nine days ago was difficult to reconcile.

For decades the peshmerga had been the force they could depend on to keep their territory safe and keep alive their dreams of autonomy.

But in one humiliating weekend, the Kurdish forces were chased back to Irbil by an enemy that has now turned its attention and guns on everything the Kurds have built during decades of war with Saddam Hussein, then the US-led invasion, and eight years of enmity with Iraq's central government, when many in the north turned inwards and quietly got rich.

In the days since the Islamic State (Isis) started storming towards Irbil on 3 August, the peshmerga has faced a reckoning. To many Kurdish officials, some peshmerga units seem built on past glories rather than current capabilities.

Several senior officials fear that Iraq's most formidable military is over-stretched as it tries to combat Isis jihadists over a 600 mile frontline.

Others say training has not kept up with demands and that the reserve force of men who have fought various campaigns have never before faced such a formidable foe.

"We have very brave peshmerga," said Masrour Barazani, chancellor of the Kurdish region security council. "But they were outgunned," he said of the clashes that led to them withdrawing from the minority areas that have now been overrun. "They had worn out light machine guns. We don't have any armaments to counter what Isis is carrying."

A flush of weapons delivered by the US on Monday has eased immediate fears of light arms shortages. But the new rifles and bullets are no match for the heavy weaponry carried by Isis, most of which was also supplied by the US – to the Iraqi military during the nine-year occupation.

Much of those heavy weapons, including tanks, humvees, troop carriers and artillery pieces were seized by Isis when the Iraqi Army abandoned all its bases in the Arab north of the country in mid-June.

The enormous arsenal has given Isis an added potency that continues to startle the Kurds and expose the limitations of their military and political power.

"I don't care if we get tanks from the devil," said Manzer Jalal, a volunteer Kurdish fighter and former peshmerga member on a frontline near Irbil this week. "We will fight them with whatever it takes, but I don't mind telling you how happy I am about the US jets."

That US air strikes have likely saved Irbil in the short term is being stated as fact among many Kurdish officials who were rattled by how close the city and key oil fields nearby came to being seriously threatened by Isis.

"Constructing a modern military to deal with this in the long run is the next step," an official said. "The US had been happy enough for us to fight alongside them and for us to remain a competent force. But they didn't want us to have weapons with which we could threaten our neighbours.

"Well, we don't need them to threaten anyone. We need them to defend ourselves. This is much more of a blow to us than Maliki's army could ever have been."

At the frontline, middle-aged men in traditional Kurdish dress were carrying frayed and chipped rocket-propelled grenades slung across their back. "I fought Saddam and the Iranians," said one man. "I fought Maliki," said a man standing next to him."

A third man, Sabah Khereji, a former translator with the US military stepped forward with an American-issued weapon.

"I've already fought these guys," he said of the jihadists only 2km away. "We're all going to need more than what we have now."