Whoever wins the coming battle for northern Syria will go a long way towards victory in a war now driven by regional agendas

The last road into rebel-held east Aleppo carves though a mile-wide paddock between an abandoned village and a looming ridgeline. Behind each of them Syrian troops advance slowly, hidden from view.

Trucks, cars and motorbikes bump through the green field, gouging deep muddy holes that are starting to resemble trenches, before joining a gravel path along a sand berm that shelters the final passage into what is left of the city.

After two-and-a-half years of war, the Aleppo at the end of the makeshift road is a wasteland where only gunmen, soldiers and a few desperate civilians now tread. Those who dare do so tentatively, knowing that the defining fight for one of the cradles of civilisation is now imminent.

Whoever wins the coming battle for northern Syria will go a long way towards victory in the war that has levelled much of the country and set the neighbourhood ablaze, threatening borders drawn a century ago and shattering several millennia of co-existence from the Mediterranean coast to Iraq’s Ninevah plains.

At the foot of the ridge just over a mile from where Syrian forces and militias have now massed, dogs chew through piled rubbish. A short distance away, rebel fighters have also dug in, using nearby towns and villages as vital supply lines while the city itself is slowly being strangled.

Black acrid smoke shrouds an industrial area on the northeastern outskirts of Aleppo, once a bustling hub of the Syrian economy. All that is left now is a ruined and desolate landscape of factories and office blocks, where exhausted fighters from both sides hide.

“The regime thinks they are going to do this easily,” said a rebel leader from the Islamic Front, the largest remaining opposition group in northern Syria, as his clapped-out car slipped through the mud. “But their army hasn’t done it so far and we’re not going to let them now.”

Rebel reinforcements are steadily making their way – through the same gouged field – to secure the last route in and out of Aleppo. Since February this year, the regime has remained strong in the north-east and north-west of the city, but has been unable to close the circle – a move that would besiege Aleppo, condemning it the same fate as a second rebel bastion, Homs, most of which was seized by loyalist troops this year.

Directly to the north, the opposition has opened a fight for the Shia town of Zahraa, where up to 15,000 locals have remained unharmed throughout the war, protected mainly by Shia militias led by Hezbollah forces from Lebanon.

On both sides, what started as a battle for control of a sovereign state has now been eclipsed by regional agendas. The almost exclusively Sunni opposition believes it is battling a Shia Islamic hegemony led by Iran and abetted by regimes in Damascus and Baghdad. Syria and Iraq, meanwhile, insist the Islamic Front and other rebel groups are no different from the ruthless Islamic State (Isis), which is trying to carve a self-governed caliphate from the ruins of both nation states.

The fight for Zahraa, one of the few Shia enclaves in northern Syria, is being led by the al-Qaida-aligned Jabhat al-Nusra, with whom the Islamic Front have an understanding but no formal alliance. After barely holding ground for much of the past year, al-Nusra recently seized large chunks of territory near the Turkish border, reasserting itself as a power player at the expense of non-jihadist groups. The fast-changing dynamic is forcing a new reckoning with the Islamic Front, which says it has waited fruitlessly for help from Arab states that was promised but never delivered.

Al-Nusra Front fighters near Aleppo. Photograph: Fadi al-Halabi/AFP/Getty Images

“There is a military reason for this fight on Zahraa,” said an Islamic Front leader, sitting in a frigid empty villa that his forces use as a command post. “We know that the regime fights with large numbers of militias – all of their victories have come from their proxies, not from their own soldiers. Those militias will run to Zahraa to defend the Shias. The regime won’t be able to move around Aleppo without them.”

In the past month, Islamic Front fighters say they have captured three Shia militiamen from Afghanistan, whom they say were brought to Syria by Iran to fight for Bashar al-Assad. They showed videos and photographs to support the claim and said others had been killed in battle.

“One of them told us that he was in prison in Afghanistan and was told that he would be freed if he learned how to fight,” the leader said. “He was taken to Iran for a 30-day course and brought here. We eventually found one of our guys who speaks basic Farsi and he could talk to him.”

Across Syria, Shia militias coordinated by Iran have risen while the influence of regime forces has ebbed. “Hezbollah are the most powerful and the Syrian army takes orders from them,” said a local leader on the eastern edge of Aleppo, where regime scouts probe most nights. “The Iraqis were also here, along with the Houthis from Yemen, but they have all gone home [to fight wars]. We can deal with what’s left.”

The fighters placed a plastic sheet on the floor and covered it with plates of falafel, salads and mezze and cups of tea. They then laid out their despair over another recent event in the war that has not gone their way: the intervention of the US air force and a coalition of Arab allies against Isis, which the rebels of the north had hoped would also clip the wings of Assad’s air force.

In January this year, the Islamic Front launched a campaign against Isis forcing it to flee Aleppo and the countryside to the north, as well as Idlib province to the west. The six-week fight left more than 2,000 Islamic Front fighters dead and, while the opposition was distracted, gave the regime an opportunity to manoeuvre to just shy of where it now threatens the north-east of the city.

Syrian forces north of Aleppo. Photograph: George Ourfalian/Reuters

Since then the regime’s incremental gains have been hard fought, with most inroads being pushed back by rebel fighters and locals, both still reeling from their losses of manpower in the war with Isis. Meanwhile Isis has lurked 20 miles away, taunting the Islamic Front with a radio station it has set up that regularly plays Islamic chants insulting the group’s members.

“They were strategic [losses] for us,” said the Aleppo commander of the gains by Isis. “And [yet] the Americans doubt our commitment to fighting them? When [the US] came back to Syria, we thought the least they could do is to stop Assad’s air force from flying. But they have bombed the city more than at any time before the Americans arrived. Of course we believe they have a deal with the regime. It is obvious.”

The Assad military’s conventional weapons have taken a savage toll all around the area where the men are gathered. Barrel bombs dropped from helicopters high above have ruined more than 30 apartment buildings. Scud missiles and other ballistic rockets have left enormous craters in their wake. Destruction is heaviest around the eastern and northeastern fringes, areas the regime will use to re-enter the city if it makes good its threat.

“Let them try,” said Mahmoud Zaher, a young rebel from the Aleppo countryside. Zaher is one of a large number of the rural poor who make up the opposition’s fighting forces from Darkoush on the Turkish border to al-Bab 25 miles east of Aleppo – the westernmost point of the swath of land across Syria and Iraq now controlled by Isis.

Isis’s next moves loom large in the reckoning of the defenders of Aleppo. So too do Jabhat al-Nusra’s. Buoyed by a month of stunning successes in Idlib province, in which they had ousted the Saudi-backed mainstream rebel group the Syrian Revolutionary Front, al-Nusra forces this week overran a regime military base in Idlib province, which had held out against other opposition units throughout the war.

Both victories give the group impetus and aegis across a fractured battlefield where both matter greatly. The Syrian Revolutionary Front had been the rival opposition group to the Islamic Front and was considered by Riyadh in particular to be a bulwark against a jihadist creep. But with its authority now diminshed, jihadists like al-Nusra and Isis are making gains in the north, while the Islamic Front is struggling to hold Aleppo.

Islamic Front fighters in Aleppo launch missiles towards the airport to try to prevent supplies from reaching regime forces. Photograph: Anadolu Agency/Getty Images

Asked before al-Nusra’s triumphs this week whether the Islamic Front would formally ally with the group, a spokesman for the Islamic Front leadership said: “These things will need to be discussed. We are on good terms and we cooperate a lot. And anyway, do you in the west expect us to sit around waiting for the Americans? We are in this situation because no one has helped us. Those who are facilitating our death and misery don’t have the moral authority to tell us how to save ourselves.”

With a fresh haul of heavy weapons seized from the Idlib base, a combined jihadist-Islamic Front effort would pose a new obstacle to the regime’s efforts to close the gap around Aleppo. But it may also invigorate UN-led moves towards a ceasefire in the north, which the opposition had been unwilling to embrace from a position of weakness.

“Assad only wants to go for this because he thinks he is winning,” said the Islamic Front spokesman. “We know we have to unite to be strong.”

In Aleppo’s Old City, where every one of the few thousand locals who have remained appears fatigued by relentless misery, talk of a ceasefire is common. “We can’t make it a surrender though,” said Saleh Homsi, sipping bitter coffee at a roadside stall. “They have to accept that they can’t win first,” he added of the regime.

Across the ridgeline at the entrance to the city, there was movement throughout the week. Syrian troops would appear first on the top then halfway down the facing slope before retreating under fire. An abandoned regime position had crumbled into the dirt, and not far away Isis graffiti, marking the group’s brief time in charge of the area, was fading under winter rains.

Islamic Front banners now fly proudest but in the shifting fortunes for the war for the north that too could soon change.

“Somebody has to win this war,” said Samer Zeitoun, a local leader. “I hope it’s us and I hope it’s soon.”

Additional reporting by Saalim Rizk.