By Leo Sands*

Syrian rebels have launched a lightning offensive against government forces in the country’s northwest, redrawing the front lines of the country’s bloody civil war for the first time in years.

The rebel groups fighting in its 13-year war Syria they are a complex patchwork of fighters, focused on fighting different enemies—including, at times, each other—and occasionally supported by foreign powers. Last week, the Islamist rebel group Hayat Tahir al-Sham (HTS) emerged as a rival Bashar al-Assadhas led Syria for almost a quarter of a century – more than half of which has been spent fighting for survival in this conflict.

The effects of the rebel advance will not only reshape the contours of the civil war, but could spread beyond Syria’s borders. The conflict could draw Russia and Iran further into the conflict, while the US has distanced itself from the ongoing offensive, calling for an urgent de-escalation. Here’s what we need to know about the main players involved in the battles.

Hayat Tahrir al-Sham (HTS)

HTS is leading the latest offensive against government forces, more than a decade after gaining notoriety at the start of the civil war as an opponent of the Assad regime. The group’s stated goal is to establish Islamic rule in Syria and is the successor to al-Qaeda’s former wing, Jabhat al-Nusra.

In recent years, HTS has used its dominance in northwestern Syria—where it had been contained by government forces—to regroup the remaining opposition forces into militant forces.

HTS also worked to tone down its image during this time. It was once linked to al-Qaida but has since distanced itself from its extremist roots, focusing instead on providing services to millions of people in Idlib province through the fledgling Syrian Salvation Government, the de facto administrators of the territory controlled by HTS. In recent statements, the group said it would protect cultural and religious sites in the Aleppoincluding churches.

The group also controls the Bab al-Hawa border crossing to Turkeya vital corridor for channeling humanitarian aid into rebel-held areas.

The US State Department has designated HTS as a foreign terrorist organization.

Syrian government forces

Government troops loyal to Assad have thwarted attempts to topple his regime since peaceful anti-government protests erupted in 2011. As Assad’s forces violently suppressed the protests, they turned into a regular insurgency, shaping the contours of the current conflict.

By 2020, government troops – backed by Iran, Russia and Hezbollah – had contained opposition rebels to a corner of northwestern Syria. Russia has been effectively acting as Assad’s air force since 2015.

Last week, government forces suddenly appeared to have lost the upper hand, with rebels gaining control of much of Aleppo, the major Syrian city that Assad’s troops recaptured in 2016. The regime’s military said it was redeploying troops from areas it controls in the provinces of Aleppo and Idlib, with the help of joint Syrian-Russian bombing air forces.

On Sunday, Iran’s foreign minister traveled to Damascus in a show of support for the Assad regime, although the nature and timing of any future aid remains unclear.

In an interview with NBC on Sunday, US national security adviser Jake Sullivan said the timing of the rebel advance was linked to the weakening of Assad’s key backers, Russia, Iran and Hezbollah, in conflicts elsewhere in the region and beyond that.

Syrian National Army

The Syrian National Army (SNA) is a coalition of Turkish-backed forces that have also been involved in recent battles, mainly against Kurdish fighters in northern Syria. These forces have also previously fought the Assad government, Islamic State militants as well as HTS and its predecessor group.

Based in northern Syria, along parts of the shared border with Turkey, SNA factions are largely composed of Syrian Arab fighters, including those who belonged to the rebellion’s first rebel group, the Free Syrian Army.

Turkey’s state-run Anadolu Agency reported that the SNA was involved in the recent rebel offensive, claiming to have seized a military airport in Aleppo.

The Turkish proxy force has also fought against allies of the US-backed Syrian Kurdish fighters, whom Turkey considers terrorists because of their links to the Kurdistan Workers’ Party (PKK), a militant group that has launched attacks inside Turkey in the name of Kurdish nationalism. In the past, UN experts have accused SNA fighters of widespread violence, including summary executions, beatings, kidnappings and looting in areas under Turkish control.

Kurdish forces

The Syrian Democratic Forces (SDF) is a multi-ethnic and multi-religious Kurdish-led coalition based in northeastern Syria that is backed by the US in the fight against the Islamic State group.

In addition to their battles against Islamist extremists, the SDF is waging a parallel conflict against Turkey and Turkish-backed militants.

Last week, the SDF reported fighting to contain advancing Turkish-backed fighters involved in the latest attack. Turkey opposes the SDF because of its ties to the PKK and has long viewed their presence near the Turkish border as a threat.

Analysts say the timing of the attack coincided more broadly with the weakening of supporters of the Assad regime. “This is about geopolitical opportunity,” said Emile Hokayem, senior fellow for Middle East security at the International Institute for Strategic Studies. “The resistance as a whole had been regrouped, re-armed and re-trained for this sort of thing.”

*Leo Sands is a reporter and breaking news editor for the Washington Post, covering news unfolding around the world.