Rebel groups fighting to depose President Bashar al-Assad of Syria battled regime forces on the outskirts of the strategic city of Homs on Saturday as they pushed toward the capital, Damascus, according to the rebels and a war monitoring group.

Advances by a coalition of opposition groups headed by the Islamist faction Hayat Tahrir al-Sham have abruptly changed the landscape of Syria’s civil war after a long stalemate. Their lightning offensive poses the most direct challenge to Mr. Assad’s power in years and is raising fears of chaos if his authoritarian government loses control over large swaths of the country.

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.

Here’s a guide to understanding the conflict, even as it changes rapidly.

What is the situation on the ground?

In just over a week, Syrian rebel forces have seized much of Syria’s northwest from the government in a fast-moving attack, upending the stalemate in the civil war. After capturing most of the major city of Aleppo last week, the rebels drove government troops from the western city of Hama on Thursday. They are now threatening the strategic city of Homs, and edging closer to the capital, Damascus.

Homs sits at the crossroads of major highways, including one that leads to Damascus. Losing it would remove a key buffer between rebel-held areas in the northwest and Damascus further south.

In the east, in another sign of the Assad government’s loosening control, a Kurdish-led force said it had deployed in Deir al-Zour, which the government had previously held. And beyond the main rebel advances, the government appeared to be losing other pockets of territory. The Syrian Observatory for Human Rights, a British-based war monitoring group, said the city of Sweida, south of Damascus, was no longer under government control.

Who is fighting?

The Syrian government

The Syrian government, led by Mr. al-Assad, has been central to the protracted and devastating civil war that began in 2011. Mr. al-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.

Mr. al-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, leaving Mr. al-Assad vulnerable.

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 jihadists formed the Al Nusra Front to fight pro-Assad forces with hundreds of insurgent and suicide attacks.

The group had early links to the Islamic State, and then to Al Qaeda. 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.

Kurdish forces

Forces from Syria’s Kurdish ethnic minority became the United States’ main local partner in the fight against the Islamic State in Syria, under the banner of the Syrian Democratic Forces. After the extremist group was largely defeated, the 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,” Mr. 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 Mr. 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, 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 Mr. al-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 President Bashar al-Assad in power.

United States

The U.S. 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 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. President Donald J. Trump withdrew many of those forces in 2019, 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.

What would a rebel victory mean?

The rebels say their goal is to oust Mr. al-Assad and it is not clear what would happen if he were to fall. Many in the international community had come to grudgingly accept him as Syria’s leader, even after he violently crushed his country’s opposition and used internationally banned chemical weapons.

For many countries, Mr. al-Assad offered a semblance of control, while a rebel takeover threatened more uncertainty in a region already in upheaval. Some Arab states last year began to normalize diplomatic relations with the Assad regime after shunning his government for years.

Publicly, American officials have been cautious about Hayat Tahrir al-Sham. But inside the U.S. 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 jihadist organization.

Whether the rebels succeed or not, the main regional players — Israel, Iran and Turkey — all have a stake in the outcome of this war, which means that the ripples will affect not just the Middle East, but also global powers like the United States and Russia.

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 started when Syrians rose up peacefully against the government of President Bashar al-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 seized parts of Syria and Iraq in 2014 and declared that territory its “caliphate,” further destabilizing the region.

Foreign interventions. Al-Assad has received vital support from Iran and Russia, as well as the Lebanese militant group Hezbollah. The rebels were backed by the United States and oil-rich Arab states like 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 al-Assad have committed by far the most atrocities. The regime has turned to chemical weapons, barrel bombs and starvation to force Syrians into submission.