Explained | How to understand Syria's rapidly changing civil war

A coalition of opposition groups in Syria, headed by the Islamist faction Hayat Tahrir al-Sham, has upended Syria's civil war after a long stalemate. Their lightning offensive, taking over several key cities in just days, is the most direct challenge to President Bashar Assad's power in years and may signal the end of his rule.

The Syrian civil war started 13 years ago, beginning during the Arab Spring and escalating into a bloody, multifaceted conflict involving domestic opposition groups, extremist factions and international powers, including the United States, Iran and Russia. More than 500,000 Syrians have died, and millions more have fled their homes.

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Here's a guide to understanding the conflict.

What is the situation on the ground?

In just over a week, Syrian rebel forces seized much of Syria's northwest from the government in a fast-moving attack. First, the rebels seized Syria's largest city, Aleppo, then days later blazed through Hama and the strategic city of Homs. On Sunday, they reached Syria's capital, Damascus.

Who is fighting?

The Syrian Government

The Syrian government, led by Assad, was central to the protracted and devastating civil war that began in 2011. Assad, who took power in 2000, is part of the family that has run Syria since a 1970 coup. They are Alawites, a minority sect that is an offshoot of Shiite Islam.

Assad initially portrayed himself as a modern reformist, but he responded to peaceful protests during the Arab Spring with brutal crackdowns, sparking a nationwide uprising.

After several years of war, the Assad government clawed back much of the territory it lost to rebels with the help of Iran, Russia, and Lebanon's Hezbollah militia. But those allies have recently been decimated or distracted by other conflicts.

Hayat Tahrir al-Sham

Hayat Tahrir al-Sham, whose name means Organization for the Liberation of the Levant, began to form at the beginning of Syria's civil war, when jihadis formed the Nusra Front to fight pro-Assad forces with hundreds of insurgent and suicide attacks.

The group had early links to the Islamic State group, and then to al-Qaida. But by mid-2016, the Nusra Front tried to shed its extremist roots, banding together with several other factions to establish Hayat Tahrir al-Sham. The United States and other Western countries still consider it a terrorist group.

The group's leader, Abu Mohammad al-Golani, told The New York Times his primary goal was to "liberate Syria from this oppressive regime." He has tried to gain legitimacy by providing services to residents in his stronghold of Idlib.

Publicly, US officials have been cautious about Hayat Tahrir al-Sham. But inside the US government, some officials believe the group's turn toward pragmatism is genuine, and that its leaders know they cannot realize aspirations to join or lead the Syrian government if the group is seen as a jihadi organization.

Kurdish Forces

Forces from Syria's Kurdish ethnic minority became the United States' main local partner in the fight against the Islamic State group in Syria, under the banner of the Syrian Democratic Forces. After the extremist group was largely defeated, Kurdish-led forces consolidated control over towns in the northeast, expanding an autonomous region they had built there. But Kurdish fighters still had to contend with their longtime enemy, Turkey, which regards them as linked to a Kurdish separatist insurgency.

There are also many other Syrian militias fighting with their own agendas and allegiances.

What about foreign powers?

Turkey

Since the beginning of the civil war, the Turkish military has launched several military interventions across the border into Syria, mostly against Syrian Kurdish-led forces. Turkey now effectively controls a zone along Syria's northern border.

Turkey also supports factions such as the Syrian National Army, a coalition of armed Syrian opposition groups. Analysts say it probably gave tacit approval to the offensive led by Hayat Tahrir al-Sham.

President Recep Tayyip Erdogan of Turkey on Friday issued a qualified approval of the rebel advance. "Idlib, Hama, Homs, and the target, of course, is Damascus," Erdogan told reporters following Friday prayers in Istanbul, according to Turkish state media. "The opposition's march continues. Our wish is that this march in Syria continues without incident."

Russia

Throughout Syria's civil war, Russia has been one of Assad's most loyal foreign backers, sending Russian troops to support his forces and jets to bomb his enemies. It has maintained a strategic military presence in Syria with air and naval bases, which it uses to support military operations in the region.

Because of the grinding war of attrition in Ukraine, analysts say Russia has been unable to support Syria's government as forcefully as it has in the past. Russian airstrikes that attempted to slow the rebel advance have been relatively sparse.

Iran and Hezbollah

Syria is a core part of Iran's "Axis of Resistance," a network of countries and groups that includes Hezbollah, Palestinian militant group Hamas and the Houthis in Yemen that hopes to destroy Israel and reduce American influence in the Middle East.

Iran smuggles weapons to Hezbollah across Iraq and Syria. Iran and Hezbollah have repaid the favor by sending thousands of militants to fight on Assad's side during the civil war.

On Friday, Iran began to evacuate its military commanders and personnel from Syria, according to regional officials and three Iranian officials, in a sign of Iran's inability to help keep Assad in power.

United States

The US role in the Syrian civil war has shifted several times. The Obama administration initially supported opposition groups in their uprising against the government, providing weapons and training, with limited effect.

After the rise of the Islamic State group in 2014, U.S. forces fought the terrorist group with airstrikes and assistance to Kurdish forces, and then stayed in northeastern Syria to prevent a resurgence. In 2019, then-President Donald Trump withdrew many of those forces, but the United States still maintains a force of about 900 troops, centered in Kurdish-controlled oil-drilling areas in the northeast and a garrison in the southeast near Syria's borders with Iraq and Jordan.

Israel

Israel's military activities in Syria have been mostly focused on airstrikes against Hezbollah and Iranian targets, especially senior military personnel, weapons production facilities and the transport corridor that Iran uses to send weapons to Hezbollah.

An Enduring Conflict

The Syrian war began in 2011 with a peaceful uprising against the government and spiraled into a complex conflict involving armed rebels, extremists and others.

The origins: The conflict began when Syrians rose up peacefully against the government of Assad. The protests were met with a violent crackdown, while communities took up arms to defend themselves. Civil war ensued.

Other groups became involved. Amid the chaos, Syria's ethnic Kurdish minority took up arms and gradually took territory it saw as its own. The Islamic State group seized parts of Syria and Iraq in 2014 and declared that territory its "caliphate," further destabilizing the region.

Foreign interventions. Assad has received vital support from Iran and Russia, as well as Hezbollah. The rebels were backed by the United States and oil-rich Arab states such as Saudi Arabia. Turkey also intervened to stop the advance of Kurdish militias.

The toll. The war has killed hundreds of thousands of people and displaced millions. Forces loyal to Assad have committed by far the most atrocities. The regime has turned to chemical weapons, barrel bombs and starvation to force Syrians into submission.