The 13-year civil war in Syria has roared back into prominence with a surprise rebel offensive on Aleppo.
The push – which saw rebel fighters seize large parts of the country’s second-largest city – is among their strongest in years, in a war whose destabilising effects have rippled far beyond Syria’s borders.
It marks the first opposition attack on Aleppo since 2016, when a brutal air campaign by Russian warplanes helped Syrian president Bashar al-Assad retake the city. Interventions by Russia, Iran, Hezbollah and others have allowed him to remain in power within the parts of Syria under his control.
As the fighting continued on Saturday, sources among the rebels told Reuters that they had also captured the city of Maraat al Numan in Idlib province, bringing all of that province under their control, in what would be another significant blow to Mr Assad.
The Syrian military sought to insist on Saturday that the insurgents had not been able to establish fixed positions in Aleppo due to the army's continued bombardment of their positions. Yet the army later said it had conducted a “temporary troop withdrawal” in Aleppo to prepare for a counteroffensive.
Sky News reported that Russian and Syrian jets had been bombing rebel positions in Aleppo on Saturday, amid expectations in Damascus that additional Russian military aid was on its way.
The surge in fighting has raised the prospect of another violent front reopening in the Middle East, at a time when US-backed Israel is fighting Hamas in Gaza, and having just announced a ceasefire with Hezbollah in Lebanon, both of which are Iranian-allied groups.
Robert Ford, the last-serving US ambassador to Syria, pointed to Israel’s ceasefire with Hezbollah after months of Israeli strikes on Syrian and Hezbollah targets as factors providing Syria’s rebels with the opportunity to advance.
Here is a look at some of the key aspects of the new fighting:
What implications could the fighting at Aleppo have?
Mr Assad has been at war with opposition forces seeking his overthrow for 13 years, in a conflict which has claimed an estimated 500,000 lives. Some 6.8 million Syrians have fled the country over the course of the war, mostly resettling in Turkey. Nearly 1.3 million people have been granted protection in EU countries, as of last year.
The roughly 30 per cent of the country not held by Mr Assad is controlled by a range of opposition forces and foreign troops. The US has around 900 troops in northeast Syria, far from Aleppo, to guard against a resurgence by the Islamist terror group Isis.
Both the US and Israel conduct occasional strikes in Syria against government forces and Iran-allied militias. Turkey has forces in Syria as well, and has influence with the broad alliance of opposition forces storming Aleppo.
The new push comes after years with few sizeable changes in territory between Syria’s warring parties, and “has the potential to be really quite, quite consequential and potentially game-changing”, if Syrian government forces prove unable to hold their ground, said Charles Lister of the US-based Middle East Institute.
The question of whether or not Isis fighters see the offensive as an opening is among the risks raised by the development, Mr Lister said.
Mr Ford said the fighting in Aleppo would become more broadly destabilising if it drew Russia and Turkey – each with its own interests to protect in Syria – into direct heavy fighting against each other.
Amid reports of Russian jets bombing rebel targets in Aleppo, Syrian military sources told Reuters on Saturday that Damascus expected new Russian military hardware to start arriving at Russia's Hmeimim airbase, near Syria's coastal city of Latakia, in the next 72 hours.
What do we know about the group leading the offensive on Aleppo?
The US and UN have long designated the opposition force leading the attack at Aleppo – Hayat Tahrir al-Sham, known by its initials HTS – as a terrorist organisation.
Its leader, Abu Mohammed al-Golani, emerged as the leader of al-Qaida’s Syria branch in 2011, in the first months of Syria’s war. His fight was an unwelcome intervention to many in Syria’s opposition, who hoped to keep the fight against Mr Assad’s brutal rule untainted by violent extremism.
Mr Golani early on claimed responsibility for deadly bombings, pledged to attack Western forces and sent religious police to enforce modest dress by women.
He has sought to remake himself in recent years, renouncing his al-Qaida ties in 2016, disbanding his religious police force, cracking down on extremist groups in his territory, and portraying himself as a protector of other religions, including by allowing the city of Idlib’s first Christian Mass in years.
What’s the history of Aleppo in the war?
At the crossroads of trade routes and empires for thousands of years, Aleppo is one of the centres of commerce and culture in the Middle East.
Aleppo was home to 2.3 million people prior to the war breaking out in 2011. Rebels seized the east side of the city in 2012, and it became the proudest symbol of the advance of armed opposition factions.
In 2016, government forces backed by Russian airstrikes laid siege to the city. Russian shells, missiles and crude barrel bombs – fuel canisters or other containers loaded with explosives and metal – methodically leveled neighbourhoods. Starving and under siege, rebels surrendered Aleppo that year.
The Russian military's entry was the turning point in the war, allowing Assad to stay on in the territory he held.
This year, Israeli airstrikes in Aleppo have hit Hezbollah weapons depots and Syrian forces, among other targets, according to an independent monitoring group. Israel rarely acknowledges strikes at Aleppo and other government-held areas of Syria.
