What led to Syria’s 13-year civil war, and why has fighting surged again?

Syrian opposition forces made a shock advance across the northern part of the country in recent days, seizing control this weekend of most of Aleppo, Syria’s economic capital, in a stunning challenge to President Bashar al-Assad that has refocused global attention on the nation’s years-long civil war.

Since its start in 2011, when Assad cracked down on largely peaceful protests, the conflict has killed hundreds of thousands of people and displaced around 13 million more, according to the United Nations. It gave rise to the Islamic State, sucked in world powers including Russia and the United States, and carved Syria into different zones of control.

For years, remnants of the opposition huddled mainly in Idlib province, along the Turkish border, as well as in other parts of northern and central Syria. Violence there flared in October with clashes between insurgents and government forces, as well as Russian airstrikes against rebel positions. The renewed fighting came as Hezbollah, the Lebanese militant group allied with Assad, redirected some of its forces from Syria to Lebanon to try to stave off an Israeli invasion.

The sudden assault by Syrian rebels, led by the Islamist militant group Hayat Tahrir al-Sham, has rapidly redrawn the war’s front lines and threatened to destabilise the country further. Here’s what to know.

What led to the Syrian civil war, and what is happening now?

In March 2011, thousands of Syrians inspired by the Arab Spring uprisings took to the streets to protest Assad’s authoritarian government and call for democratic reforms. Assad had succeeded his father, Hafez al-Assad, after his death in 2000, extending the family’s decades-long rule. The Assads are members of Syria’s minority Alawite community, a religious sect that split from Shiite Islam. The majority of Syrians are Sunni Muslims.

The mass demonstrations in Sunni-majority cities were mostly peaceful. But the government responded with a brutal military and police crackdown, shelling civilian neighbourhoods and carrying out wide-scale arrests. Some Syrians began arming themselves to fight back, and several Islamist militant groups, some of which battled U.S. troops in Iraq, resurfaced in Syria. By June 2012, the United Nations declared that the fighting in Syria was a full-blown civil war.

After years of fighting, Syrian government forces backed by Russia, Hezbollah and Iran retook much of the territory captured by rebels. But a patchwork of groups still maintained control over pockets of the country. The U.S. military keeps troops in eastern Syria, where it helped Kurdish-led fighters vanquish the Islamic State.

The conflict had largely quieted until last week, when opposition fighters led by Hayat Tahrir al-Sham seized a government base in western Aleppo on Wednesday. They have since wrested control of Aleppo - once Syria’s largest city before some of its population migrated to Damascus, the political capital - and were headed farther south to the city of Hama.

It was unclear how much territory the rebels could hold or for how long. The Syrian military said it was mobilising forces for a counterattack.

Who are the rebel fighters, and what do they want?

Hayat Tahrir al-Sham (HTS), which is based in Syria’s Idlib province, is leading the offensive with a constellation of smaller rebel factions. Among them is the Syrian National Army (SNA), a loose, Turkish-backed coalition of forces that includes fighters who belonged to the uprising’s first rebel group, the Free Syrian Army.

HTS and the SNA have clashed in the past - but both groups want to overthrow the Assad government. For HTS, the goal is to establish Islamic rule in Syria, although it has issued statements in recent days saying it will protect cultural and religious sites in Aleppo, including churches.

The U.S. State Department designated HTS as a foreign terrorist organisation. The group is the successor to onetime al-Qaeda affiliate Jabhat al-Nusra. It is no longer associated with al-Qaeda and represents a more localised faction of Islamist fighters. The group controls the Bab al-Hawa crossing with Turkey and provides military muscle to the Syrian Salvation Government, the opposition’s de facto administration in northwest Syria.

Why did Syrian rebels launch their offensive now?

Analysts say opposition fighters have been reorganising, rearming and retraining for years to stage an attack on government forces. Their moves to regroup coincided with the weakening and distraction of Assad’s key allies, including Russia, Hezbollah and Iran.

“This has to do with geopolitics and local opportunity,” said Emile Hokayem, senior fellow for Middle East security at the International Institute for Strategic Studies. “The rebellion at large had regrouped, rearmed and retrained for something like this.”

Earlier in the conflict, Iran and Hezbollah provided Assad with crucial personnel to stave off the rebellion, eventually ousting opposition fighters from Aleppo and other parts of the country. Russia acted as Assad’s air force, striking rebel positions in towns and cities in the country’s northwest. But in 2022, Russia invaded Ukraine, and then devoted much of its military resources to battling the Ukrainian military.

Hezbollah and Iran both became quickly ensnared in the war between Israel and Hamas in Gaza that began last year. Hezbollah fought a low-simmering conflict with Israel along Lebanon’s southern border until an all-out war erupted in September, and the Israeli military pummelled the group, killed its senior leaders and invaded part of the country.

For its part, Iran engaged in tit-for-tat strikes with Israel for the first time, launching drones and ballistic missiles at Israeli territory after an airstrike killed senior Iranian commanders at Tehran’s consulate in Damascus. Israel also launched strikes inside Iran targeting its air-defence systems and missile-production facilities, knocking out critical elements of its strategy of deterrence.

All of this left Assad and his forces particularly vulnerable in Syria. “I don’t think personally that they thought they would reach Aleppo,” Jihad Yazigi, editor of the Syria Report, an online publication, said of the rebels. “I think they certainly knew that the regime was weaker and they had an opportunity, and they wanted to seize this opportunity to enlarge a bit the areas under their control.”

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Kareem Fahim and Louisa Loveluck contributed to this report.

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