For six weeks, thousands of Iraqi Shia Turkmen were under siege by militants of the Islamic State (IS) in a small town in northern Iraq, where the radical Sunni group had been trying to take over one of the last unoccupied areas of north-central Iraq after its recent military advances.

But Amerli’s steadfastness against the terror group is not just about the courage of its population. It also underlines the shift in Iraq’s ethno-sectarian battle lines that has been triggered by IS’s partial defeat.

Amerli is less than eight km from the Kurdish Peshmergas who have been receiving US military assistance to stave off the IS advance into the Kurdish-controlled region. Neither the Peshmergas nor the Americans initially showed much willingness to take on the IS in Amerli and save the town’s 20,000 people from a murderous onslaught.

Meanwhile, Turkey, Iraq’s northern neighbour, although traditionally considered a protector of the Turkmen, an ethnic Turkish minority, has taken a do-nothing approach to stop the calamity-in-the-making.

The Iraqi security forces, recovering from a humiliating defeat by IS and unable to stop its advances, repeatedly tried to relieve Amerli but to no avail.

Feeling abandoned, the town’s poorly armed population organised to fight the IS extremists closing in on the town. To many Iraqis these people’s heroic resistance in scorching summer heat, with little drinking water, food or other necessities, has evoked the memory of the siege of Stalingrad during World War II.

It was the Iran-backed Shia militias who eventually did the job of breaking the siege on the town and freeing its battle-weary population on Sunday, demonstrating combat skills superior to those of the government and Kurdish troops who fell back to their territories after an IS push in June.

In the aftermath of this impeccably executed offensive, those who had previously abandoned Amerli to its fate during the siege claimed credit for the breakthrough. The Iraqi army and Kurdish Peshmerga were trumpeted as having taken part in the victory, while the American media claimed it was only made possible by US air strikes.

Turkey, meanwhile, was praised for sending humanitarian aid.

The siege and the town’s liberation underlines the multi-dimensional aspects of the Iraqi conflict, in which many ethno-sectarian domestic forces and regional and international actors are involved. It also underscores the importance of the victory in the context of the rise of IS and its seizure of large amounts of territory in northern and western Iraq.

Though a small farming community, Amerli is important in both military and political terms. The town, 170 km north of Baghdad, is located on a strategic plateau, the Himreen Heights, linking the Iraqi capital to the northern Kurdish-controlled territories.

Amerli also cuts off the IS-controlled areas from the east and the west. The control of the plateau by Shia forces could allow them to build up strong defences against the Sunni insurgency and push westwards, linking up with forces fighting IS in Tikrit.

The success of the campaign in Amerli has also bolstered spirits in the south, where Iraqi security forces and Shia militias are battling IS militants trying to capture more territory and topple the Shia-led government.

Shia militias are already operating in central Iraq to deter IS attacks on Baghdad and other towns, but their successes in Amerli will grant them a position in the front line of the fighting as important paramilitary units.

Undoubtedly, breaking the siege of Amerli will have far-reaching consequences for the conflict in Iraq. It will embolden the Shias and prove that Shia militias, with their growing numbers and battle-tested skills, will not be at a disadvantage in the fight.

Moreover, the victory in Amerli reveals a lot about the Shias’ strategic thinking. While the Shia-led security forces and militias have been able to slow the IS advance on Baghdad and key Shia areas, they have been less enthusiastic about liberating Sunni-populated cities.

A look at the Iraqi landscape following the IS advances shows that the Shia are using whatever skills and experience they have to draw the lines of a prospective Shia enclave in central and southern Iraq.

The IS summer offensive has already pushed Iraq close to partition between its Shia Arab majority in the south, its Sunni Arab provinces in the centre, and its semi-autonomous ethnic Kurdish region to the north.

Since the standoff with IS began in June, the Kurds have captured vast swathes of territory in the so-called disputed areas and announced that these are now part of the Kurdistan Region.

Kurdistan President Masoud Barzani has also asked the Kurdish parliament to establish an electoral commission and set a date for a referendum on independence, vowing that the newly acquired territories will never be returned to Baghdad’s control.

On the other hand, the IS takeover of huge chunks of Sunni-populated territory has also changed Iraqi Sunni politics. While IS remains at the head of the rebellion, the terror organisation has formed an alliance with other Sunni groups, including Saddam Hussein loyalists, other hard-line Islamic groups and tribal leaders.

These groups form the backbone of IS and provide it with military expertise, combat skills and local support.

In addition, non-Arab Sunnis are taking a vanguard role with IS in the fight against the Shias. The media in Iraq has reported that those besieging Amerli were Kurdish extremists belonging to the Ansar Al-Islam Group, formerly known as Ansar Al-Sunna, a Kurdish extremist group allied with IS and bent on establishing an Islamic state.

Other reports have suggested that the rapid IS advance in Turkmen areas only became possible because of the support the organisation has received from Sunni Turkmens.

Turkmen extremists have long been known to be operating with Al-Qaeda-affiliated groups, but it is particularly grim to find that they have been participating in atrocities against fellow ethnic Turks for sectarian reasons.

It is unlikely that Iraq’s Sunnis will stay allied to IS for long. But the terror group will meanwhile cause enough damage to deepen Iraq’s communal conflict by further dividing Iraqis on sectarian lines.

Something extraordinary has been happening in the Iraqi Sunni community since the IS takeover of the Sunni provinces in June. The biggest change is that most Sunnis now want to have a sort of autonomous region in Iraq that will give them more say in running their own provinces.

Set against the traditional Sunni commitment to remain in a unified Iraq, such are the risks that many Sunnis seem prepared to take in order to confront the fundamental threat of IS.

It explains why so many Sunnis have started talking about a federal Sunni region in order to end their political marginalisation and improve their living conditions through a system of sharing Iraq’s national wealth.

The proposal for Sunni self-rule has been gathering momentum with efforts to form a new government, under pressure to be more inclusive so that it can better combat IS. Another source of pressure on the present Shia-led government has been the renewed proposal for a “functioning federalism” in Iraq put forward by US Vice-President Joe Biden and repeated last week.

Though this has not been officially adopted by the US administration, the plan suggests that Iraq be divided into three semi-autonomous regions for the Shias, Sunnis and Kurds. The Shias seem to be taking this partition plan seriously and fear that the conflict triggered by IS may now have put the suggestion on the front burner.

Even before the Amerli victory, Shias were readying themselves for such an eventuality. Soldiers and militias have been busy building a network of trenches and earth barricades around hotspots such as Jurf Al-Sakhar west of Baghdad, Samara in the north and areas in Diyala to the west.

These defences seem to be intended to protect the capital and Shia-populated cities from invasion. But it also indicates that a border line is under construction to separate these areas from the Sunni heartland.

In many small towns around Baghdad Shia militias are reportedly forcing Sunni inhabitants to leave their homes under suspicion that they could be IS sleeper cells or sympathisers.

This quiet sectarian cleansing, being carried out under the eyes of the security forces, is another sign of Shia preparedness for Iraq’s breakup, which the Shias probably believe is just waiting over the horizon.