Explained | Why did Syria's civil war reignite?

Syrian opposition fighters have made their most significant advance in years against government forces, shaking up a civil war that had long been at a stalemate.

The new rebel offensive began Wednesday in Aleppo province in northwestern Syria. By Saturday, anti-government forces had captured most of the major city of Aleppo, and by Sunday, they were in control of a broad stretch of land across the provinces of Hama, Idlib and Aleppo, in the west and northwest of Syria, according to the rebel-linked administration and a British-based monitoring group, the Syrian Observatory for Human Rights.

The offensive aims to stop attacks by government forces and their Iran-backed militia allies, a rebel commander said. It is the most serious challenge to President Bashar Assad's government in years.

Government troops loyal to the president have been trying to repel the rebels, rushing reinforcements to the battlefield, launching airstrikes and getting support from Russian fighter jets, the Observatory said Sunday.

More than 300 combatants and 100 civilians have died in the recent fighting, the monitoring group said. It gathers information from a network of anti-government activists and others across Syria, and its numbers could not be verified independently.

Who are the rebels?

The offensive unites various rebel factions that represent the last vestiges of a once-sprawling array of opposition groups. Starting in 2011, they fought hard to oust Assad and, at one point, controlled large parts of the country.

The main group is Hayat Tahrir al-Sham, a faction formerly linked to the terrorist group al-Qaida. It controls most of the northwestern territory still held by opposition groups.

Several Turkish-backed rebel groups have also joined the offensive, according to commanders of the groups and the observatory.

Though they share a common enemy, the various rebel factions have often fought among themselves, undermining the cohesion they needed to challenge the Syrian military.

What are the aims of the offensive?

In a video statement announcing the offensive, Lt. Col. Hassan Abdulghany, military commander of the opposition's operations room, said the attack was aimed at stemming Syrian airstrikes and other attacks on opposition-held territory.

"To push back their fire from our people, this operation is not a choice," he said. "It is an obligation to defend our people and their land. It has become clear to everyone that the regime militias and their allies, including the Iranian mercenaries, have declared an open war on the Syrian people."

Iran has backed the Syrian government throughout the war, sending advisers and commanders of its powerful Revolutionary Guard force to bases and front lines and backing militias with thousands of fighters to defend the government.

What does the Syrian government say?

A Syrian government statement said that Assad had spoken to the leaders of the United Arab Emirates and Iraq on Saturday, vowing that Syria would "defeat the terrorists, regardless of the intensity of their attacks." Syrian officials routinely refer to rebels as terrorists.

Assad on Sunday directed Gen. Abdul Karim Mahmoud Ibrahim, the army chief of staff, to pay a field visit to Hama, where military reinforcements, including troops, equipment and weapons, were also sent, the Syrian state media reported. On Sunday the president also met with Iranian foreign minister, Abbas Araghchi, who flew to Damascus to show support for the Syrian government.

The Syrian military said in a statement on Saturday that its operation to push back the rebels was "successfully" progressing. It tried to discredit reports about rebel advances, saying that the armed groups were spreading "false news" to undermine morale.

Is this linked to the regional conflict?

While Syria has not been directly involved in the conflicts roiling the Middle East over the past year, its territory has long been a proxy battlefield for international powers.

For years, Israel has carried out deadly strikes in Syria, saying its targets are Iran-backed militants including the Lebanese group Hezbollah. Those strikes have escalated since Hamas led attacks on Israel on Oct. 7, 2023.

The Israeli military has said some of these strikes seek to cut off the flow of weapons and intelligence between Hezbollah and Iran. Weapons and money have long streamed from Iran across Syria's borders to Hezbollah in Lebanon.

In April, a deadly Israeli strike that hit part of the Iranian Embassy complex in Damascus, Syria, killed several senior Iranian commanders.

Iranian media reported Thursday that a commander of Iran's Revolutionary Guard was killed in the rebels' new offensive.

Who controls what in Syria?

More than a decade of civil war, proxy battles and an invasion by the Islamic State group have left Syria carved up into different zones of control.

The government now controls more than 60% of the country, including most major cities. But that was not always the case.

At the height of the opposition's strength in the civil war, and after the Islamic State group overran parts of Syria, the government had lost control of most of the country.

But the tide turned in 2015 when Russia's military intervened to help Assad.

Still, large parts of the Syria are out of government control, including opposition-held areas in the northwest and the northeast, which is dominated by a Kurdish-led militia backed by the United States.

The opposition-controlled area of northwest Syria includes parts of Idlib and Aleppo provinces, and is home to about 5 million people. More than half of them were displaced from their homes elsewhere in Syria.

Though the Islamic State group lost its last territorial foothold in Syria in 2019, it maintains sleeper cells believed to hide in Syria's vast desert and carry out occasional attacks on government soldiers and civilians.