In the first year or so after the fall of Saddam Hussein, it used to be a commonplace in interviews for Iraqis of different sectarian persuasions to insist not on their identity as Shia or Sunni. They would emphasise they were from such and such a city and "proudly Iraqi". What was important, I was told, was unity.

There was a sense it mattered to say it. Why would soon become clear. In the worst period of the insurgency against the US-led occupation and the dreadful sectarian conflict that followed, worst-case scenarios would be rehearsed by outside observers. Among them was the fear that Iraq could split into three constituent parts with a Kurdish state in the mountains to the north, a largely Shia one stretching down to Basra and the port at Umm Qasr and, in between, a rump Sunni entity. Even in the worst of the worst days, when bombs shook the capital's windows almost daily and sectarian murderers, including police commando units, operated with impunity, it never reached that point. Iraq stuck together, just, in a ramshackle way. Now, after a week of sudden, violent turbulence, what seemed unlikely then suddenly seems possible.

Faced with a lightning advance by a few thousand jihadi fighters from the Islamic State in Iraq and the Levant (Isis), the Iraqi army's presence in the northern and western Sunni majority provinces has collapsed. Kurdish peshmerga fighters control the oil hub city of Kirkuk, something they have long dreamed of. Iraq's weak and fragile state is spidering with cracks.

If it has seemed sudden, it is because the breathless shorthand describing the crisis has disguised a fact that Iraq has been grinding towards this moment of existential truth for the past two years at least, a path from which none of its key actors has seemed able or willing to divert it.

The roots of the crisis are deep and multi-layered. The notion that it is nothing more than the result of a continuum of chaos leading from the US-led invasion in 2003 to the current point is not only facile, it ignores the fact that conditions in Iraq had enjoyed a period of relative improvement following the high point of the bloodshed of the sectarian war in 2007-08. And its bloody unsticking in the recent period has come from a convergence of separate but connected issues.

First was the growing autocracy of prime minister Nouri al-Maliki's Shia-dominated government, which chose to promote a sectarian, rather than an inclusive, agenda.

Then there was the failure of a largely peaceful Sunni protest that started in Anbar province to effect pressure for change on an intransigent Maliki. Amid a sharp rise in bombings, the coincidence of an increasingly sectarian war in neighbouring Syria has acted as an accelerant poured on the fraught politics of Iraq.

On the first point, there is no question that Maliki bears large responsibility for manufacturing a crisis that has become more real and terrible than his own divisive rhetoric could have conceived. His appeal to sectarianism as an electoral gambit morphed into something more dangerous. Rejecting legitimate complaints of unfair trial, imprisonment and mass executions, and of systematic political and economic disenfranchisement, Maliki cut off any ability to communicate effectively with the Sunni minority's complaints.

On the Sunni side, that has had its own dangerous traction. Leaders of a protest camp movement established in Fallujah and Ramadi in December 2012, in response to the government's arrest of the bodyguards of the Sunni finance minister – and later broken up by the government – struggled as the months went on to persuade young followers to reject violence, even as a resurgent jihadi movement was growing in strength.

Where that disaffection most powerfully took hold was in Sunni-dominated cities, including Mosul, that have long been restive and unstable.

As Letta Tayler of Human Rights Watch observed correctly in Foreign Policy last week: "Long before the city's dramatic fall, Isis... and its precursor, al-Qaida in Iraq, were operating openly for years in Mosul, killing civilians... with impunity, manipulating the justice system, and even collecting so-called 'jihad taxes' from local businesses. And yet Iraq's extensive military and security apparatus did almost nothing."

Instead, it appears the army was a fig leaf in places such as Mosul, its presence nothing more than a show, amid a population that had long lost faith in it. As Charles Lister of the Brookings Institution suggested last week, all of this explains how a relatively small group of jihadi fighters managed to rampage so effectively through at least four Iraqi cities. Those fighters, Lister ventured, should be seen as much as a catalyst that triggered wider Sunni resistance than as a mark of military prowess.

If that is the background, what happens next? One thing is clear. Isis itself has insufficient manpower to hold the territory of the Iraqi cities it has taken, let alone to threaten the capital. Multiple interests both within Iraq and in the wider region, not least neighbouring Iran, and beyond will also be determined to see it driven back. A question, too, is the reach of the new sense of rebellion, not least among the middle-class Sunnis of the capital. The last time I visited Baghdad, just over a year ago, I found it a frightened and apprehensive place, fearful that the horror of its recent past might return but also desperate that should not happen. And, ironically, it would appear that those with most to lose in a worsening sectarian based-conflict and Iraq's fragmenting would be the country's Sunnis.

Despite the grandiose statements of Isis about its caliphate, in economic terms the best Sunnis could hope for would be an impoverished rump statelet, with the Kurds controlling the northern oil fields around the hub city of Kirkuk, while production from South Oil Company would remain firmly under Baghdad's control. It is hard to see, too, a scenario that would see Baghdad fall to Isis, or any combination of Isis and other groups, although more extraordinary things have happened.

Manpower – and firepower – available to any resurgent Shia militias, even excluding Iraq's security forces, would likely be more than equal to what Isis can field. More dangerous still for Sunnis is that a return to serious sectarian conflict would threaten Sunni suburbs with a return of the Shia death squads, where the definition of a "terrorist" was often simply having the wrong name or coming from the wrong suburb. And that risk was raised dramatically on Friday as Shia militia men poured north to protect the Shrine of the Golden Dome in Samarra and as grand ayatollah Ali al-Sistani called on Shias to take up jihad against Isis. If Iraq's Sunnis have legitimate complaints, their leaders, too, must take a share of the blame. Just as Maliki has been unwilling to compromise, Sunni mainstream political leaders have also dug in hard to their positions, unable and unwilling to step outside the same sheltering sense of safety in collective community grievance to reach for compromise.

Which leaves the faintest of possibilities. Over the past two years, it has been a commonplace among both Sunni political figures and advisers close to Maliki to repeat that neither side has an interest in a return to the darkest and most terrible days of Iraq's sectarian bloodletting that nightly saw bodies stuffed into drains, abandoned on the rubbish tips, people – where they congregated – slaughtered by bombs.

Staring into the abyss , it is just possible that a weakened Maliki might blink. Because in the end, Isis and the fellow jihadi travellers of its leader, Abu Bakr al-Baghdadi, are a symptom, not a cause. Like Abu Musab al-Zarqawi, al-Qaida in Iraq's previous violent emir, they are opportunistic interlopers whose vision is shared by the smallest of minorities. But whether Isis will persist or be pushed back, in an incendiary few days they have set a fire that may be far harder to put out.