Who are the rebels in Syria?

With rebel forces now inside the Syrian capital Damascus, and reports that President Bashar al-Assad has fled the country, anti-government forces appear to have brought about the end of his regime less than a fortnight after their lightning offensive began.

Islamist fighters took control of the northern city of Aleppo in late November before swinging south through Hama and Homs - areas previously under government control.

In southern Syria, close to the Jordanian border, local rebels have captured most of the Deraa region, the birthplace of the 2011 uprising against Assad.

In many instances, it is reported that the Syrian military either left their posts or defected to the opposition.

The initial attack was led by the Islamist militant group Hayat Tahrir al-Sham (HTS) - which has a long and involved history in the Syrian conflict.

HTS is designated as a terrorist organisation by the UN, US, Turkey and other countries.

HTS was set up under a different name, Jabhat al-Nusra, in 2011 as a direct affiliate of Al Qaeda.

The leader of the self-styled Islamic State (IS) group, Abu Bakr al-Baghdadi, was also involved in its formation.

It was regarded as one of the most effective and deadly of the groups ranged against President Assad.

But its jihadist ideology appeared to be its driving force rather than revolutionary zeal - and it was seen at the time as at odds with the main rebel coalition under the banner of Free Syria.

And in 2016, the group’s leader, Abu Mohammed al-Jawlani, publicly broke ranks with Al Qaeda, dissolved Jabhat al-Nusra and set up a new organisation, which took the name Hayat Tahrir al-Sham when it merged with several other similar groups a year later.

For some time now, HTS has established its power base in the north-western province of Idlib where it is the de facto local administration, although its efforts towards legitimacy have been tarnished by alleged human rights abuses.

It has also been involved in some bitter infighting with other groups.

Its ambitions beyond Idlib had become unclear.

Since breaking with Al Qaeda, its goal has been limited to trying to establish fundamentalist Islamic rule in Syria rather than a wider caliphate, as IS tried and failed to do.

It had shown little sign of attempting to reignite the Syrian conflict on a major scale and renew its challenge to Assad’s rule over much of the country - until now.

In March 2011, pro-democracy demonstrations erupted in the southern city of Deraa, inspired by uprisings in neighbouring countries against repressive rulers.

When the Syrian government used deadly force to crush the dissent, protests demanding the president's resignation erupted nationwide.

The unrest spread and the crackdown intensified. Opposition supporters took up arms, first to defend themselves and later to rid their areas of security forces. Mr Assad vowed to crush what he called "foreign-backed terrorism".

Hundreds of rebel groups sprang up, foreign powers began to take sides and extremist jihadist organisations such as the Islamic State (IS) group and al-Qaeda, became involved.

The violence rapidly escalated and the country descended into a full-scale civil war drawing in regional and world powers.

More than half a million people have been killed and 12 million have been forced to flee their homes, about five million of whom are refugees or asylum seekers abroad.

The war in Syria had for the past four years felt as if it were effectively over.

President Bashar al-Assad’s rule had essentially been uncontested in the country’s major cities, while some other parts of Syria remained out of his direct control.

These include Kurdish majority areas in the east, which have been more or less separate from Syrian state control since the early years of the conflict.

There had been some continued, though relatively muted unrest, in the south where the revolution against Assad’s rule began in 2011.

In the vast Syrian desert, holdouts from the group calling themselves Islamic State still pose a security threat, particularly during the truffle hunting season when people head to the area to find the highly profitable delicacy.

And in the north-west, the province of Idlib has been held by militant groups driven there at the height of the war.

HTS, the dominant force in Idlib, is the one that has launched the surprise attack on Aleppo.

For several years, Idlib remained a battleground as Syrian government forces tried to regain control.

But a ceasefire deal in 2020 brokered by Russia, which has long been Assad’s key ally, and Turkey, which has backed the rebels, has largely held.

About four million people live there - most of them displaced from towns and cities that Assad’s forces won back from rebels in a brutal war of attrition.

Aleppo was one of the bloodiest battlegrounds and represented one of the rebels’ biggest defeats.

To achieve victory, President Assad could not depend on the country’s under-equipped and poorly motivated conscript army alone, which soon became dangerously stretched and regularly unable to hold positions against rebel attacks.

Instead, he came to rely heavily on Russian airpower and Iranian military help on the ground - mainly through militias sponsored by Tehran.

These included Hezbollah.

There is little doubt that the setback Hezbollah has suffered recently from Israel’s offensive in Lebanon, as well as Israeli strikes on Iranian military commanders in Syria, has played a significant part in the decision by jihadist and rebel groups in Idlib to make their sudden, unexpected move on Aleppo.

In the past few months, Israel has intensified its attacks on Iranian-linked groups as well as their supply lines, inflicting serious damage on the networks that have kept these militias, including Hezbollah, operative in Syria.

Without them, President Assad’s forces have been left exposed.

Additional reporting by Maia Davies.