After five days of siege and foreboding, the citizens of Baghdad breathed easier on Saturday. Old-world tea houses were once again brimming. So were new militia recruitment centres, where would-be fighters signed up to defend the capital.

The city's collective relief stemmed from three live television addresses, only one of them made by an Iraqi. On Friday, President Barack Obama said enough to convince most that he would soon send US jets to deal with the insurgents at the gates. Hours later the Iranian president, Hassan Rouhani, said an alarmed Iran, which is overwhelmingly Shia, would send whatever it took to stem the insurgent Sunni tide. The alliance of common interests was perceived as a rebuff to Isis (the Islamic State in Iraq and the Levant), a jihadist group so hardline it was disowned by al-Qaida. Isis has been rampaging through the country, pledging to rewrite the region's borders.

But it was a religious cleric who succeeded in steeling Iraqis for a fightback. The call to arms by the highest Shia authority in the land, Ayatollah Ali al-Sistani, mobilised in less than one day around a division of militia men who, unlike the military, will not run from a fight with the insurgents.

Help is clearly on the way. And it could not come soon enough for many. "I heard the US president speak and his words seemed reasonable," said tribal leader Sheikh Abu Wissam al-Saade at a recruitment centre in the eastern suburb of Karrada, as three new volunteers arrived wanting to fight. "I would say one thing, though, he is partly to blame for this mess. He's been refusing to send us fighter jets for the past six years, because [Kurdish leader Massoud] Barzani has warned him not to."

One scrawny recruit signing on with his brother said: "I have come to fight for the sake of my country. This place has been through a lot."

Karrada wears the scars of insurgency more than most places in Baghdad, with up to five bombs a month shattering its glitzy shopfronts since US forces withdrew. Ground zero of the current threat, however, is around 60 miles to the north. Here, and in the city's vulnerable western approaches, the new volunteers are already preparing to confront Isis.

Iraq's immediate future seems sure to be determined by non-state actors and powerful foreign patrons. So much for the state, which has done little more than reel in horror ever since the fall of its three largest cities to a small insurgent force in less than one week.

"With the scale of this crisis, all of the military units and the militia groups have melted into each other," said al-Saade. "The fighting force against Da'ash [Isis] has become a common entity."

Those in the provinces contacted by the Observer revealed that they had already dispatched upwards of 15,000 men – most without training – to Baghdad or beyond. Dhi Qaa Yehya Mohammed Bakr said his province had already sent 2,500 fighters to Baghdad, while 25,000 have signed up around the province. Columns of minibuses, some escorted by police trucks, snaked along almost empty highways from the south all day on Saturday.

Signs of Shia fervour once again blending with militancy were commonplace in Baghdad . Military trucks carrying tonnes of ammunition pulled out of bases waving flags depicting the revered Shia martyrs, Hussein and Abbas. There was no sign of the Iraqi banners that were only a week earlier a far more frequent sight.

As their fightback begins, Baghdad and the south are openly rallying around sectarian symbols rather than a national cause. Nevertheless, the rapid response to Sistani's call to arms, and the comfort drawn from the prospect of imminent heavyweight firepower, has transformed the jihadist push into a counter insurgency. In one extraordinary week, Iraq has gone from relative safety to mortal danger. It now teeters somewhere in between.

Lost in the tumult between Sunni insurgents and Shia militias has been the moment of reckoning for the Kurds of the north. As the week closed, the extent of the Kurds' moves to capitalise on the central government crisis was becoming clear. The dramatic entry of the Kurdish Peshmurga forces into the disputed city of Kirkuk changed the balance of power between the seat of Kurdish power, Erbil, and Baghdad. Just as significant was the Peshmurga's move into other disputed areas, nearly as far south as Baquba.

One Iraqi army sergeant stationed in Jalula in central Iraq said that he was shocked as the Peshmurga rolled into his base last Thursday.

"They arrived at the battalion and talked to my commander and told him to surrender the whole unit's equipment and ammunition. They said they had spoken with the brigade commander. They said 'Da'ash is on its way and it will be your responsibility'.

"My commander called the brigade and he told them to give them what they wanted. He said he had spoken to the division leaders.

"They arrived in civilian cars and packed more weapons in the trunks, and they filmed us with their cellphones as we left. Then they raised the Kurdish flag over our base. Overnight the Kurds became a state."

Other soldiers and officers claimed similar curious capitulations were repeated in other bases in the centre and north of the country.

"This is a festival for the Kurds," said a second man back at the Karrada recruitment centre. "We are in this situation because of them. They are greedy and totally self-interested. That doesn't surprise me, but the collaboration of the Iraqi commanders is truly a shock and needs to be explained."

The besieged Iraqi leader Nouri al-Maliki seems unlikely to be able to do that. Some of the accused generals are reported to have moved back into their homes in the green zone. Others have taken refuge with the Kurds.

What remains of the Iraqi military is preparing to defend Baghdad. But the re-emergence of the militias, and of the enigmatic leader of the Iranian Revolutionary Guards Quds Force, Qassem Suleimani, who arrived in Baghdad on Friday, probably implies that they will not play a dominant role.

Hassan Rouhani's suggestion that Iran and the US could work together to defeat Isis is a new twist on "my enemy's enemy is my friend", but one that is sure to make life difficult for the insurgent group in the weeks and months ahead.

"They [Isis] have a very effective media arm," said a senior Iraqi official. "They could actually teach the west about the art of propaganda. But when a real force stands and fights them – as they will in Baghdad – they cannot take this city."

Iraqi tribal leaders and officials are also banking on Isis being unable to maintain the hearts-and-minds approach it has adopted with the Sunni towns and cities, some of which appears to have paid off for them.

"You tell me what you would do if you were me," said Mustafa al-Rai, a mechanic from Mosul who has stayed in the city. "Maliki's people and military were persecuting us. They were treating us like we were rubbish on their soles. Now we have Sunnis coming to town promising to change that. They don't share my beliefs, but they are being reasonable to us."

Iraq's Sunnis have already encountered a previous incarnation of Isis, which from 2004-07 ran roughshod over Anbar province and much of Mosul. With the help of the US army, tribal leaders ousted them, and for several years they remained defeated. The Syrian civil war changed that, proving to be a lightning rod used to reignite their ambitions. And throughout 2013 the group was again ruthlessly imposing its will over northern and eastern Syria. This year a local uprising ousted them once more. "What makes you think it will be different this time?" asked al-Rai.

"They will soon reveal their colours. And by then, the new friends [Iran and the US] may have helped us out."

Additional reporting by Yousif al-Timimi