What to know about sudden rebel gains in Syria's 13-year war and why it matters

WASHINGTON — The 13-year civil war in Syria has roared back into prominence with a surprise rebel offensive that captured Aleppo, one of Syria's largest cities and an ancient hub of Middle East culture and commerce. The push is the rebels' strongest in years in a war whose destabilizing effects have rippled far beyond the country's borders.

It was the first opposition attack on Aleppo since 2016, when a brutal Russian air campaign retook the northwestern city for Syrian President Bashar Assad after rebel forces had seized it. Intervention by Russia, Iran and Iranian-allied Hezbollah and other groups has allowed Assad to remain in power within the 70% of Syria under his control.

Insurgents led by the jihadi group Hayat Tahrir al-Sham launched the two-pronged attack on Aleppo last week and moved into the countryside around Idlib and neighboring Hama province. The Syrian military and its foreign allies have rushed reinforcements and launched airstrikes as they attempted to stall their momentum.

The surge in fighting has raised the prospect of another violent front reopening in the Middle East, at a time when U.S.-backed Israel is fighting Hamas in Gaza and Hezbollah in Lebanon, both Iranian-allied groups.

Robert Ford, the last-serving U.S. ambassador to Syria, pointed to months of Israeli strikes on Syrian and Hezbollah targets in the area, and to Israel's ceasefire with Hezbollah in Lebanon last week, as factors providing Syria's rebels with the opportunity to advance. Russia, Assad's main international backer, is also preoccupied with its war in Ukraine.

Here's a look at some of the key aspects of the new fighting:

Why does the fighting in Aleppo matter?

The long war between Assad and his foreign backers and the array of opposition forces seeking his overthrow has killed an estimated half-million people and fractured Syria. It started as one of the popular uprisings against Arab dictators in the 2011 Arab Spring, before Assad's crushing of what had been largely peaceful protests turned the conflict violent. Some 6.8 million Syrians have fled the country since then, a refugee flow that helped change the political map in Europe by fueling anti-immigrant far-right movements.

The roughly 30% of the country not under Assad is controlled by a range of opposition forces and foreign troops. The U.S. has about 900 troops in northeast Syria, far from Aleppo, to guard against a resurgence by the Islamic State group. Both the U.S. and Israel conduct occasional strikes in Syria against government forces and Iran-allied militias. Turkey has forces in Syria as well, and has influence with the broad alliance of opposition forces storming Aleppo.

Coming after years with few sizeable changes in territory between Syria's warring parties, the fighting ''has the potential to be really quite, quite consequential and potentially game-changing,'' if Syrian government forces prove unable to hold their ground, said Charles Lister, a longtime Syria analyst with the U.S.-based Middle East Institute.

Risks include if militants with the IS see the renewed fighting as an opening, Lister said. The IS, a violently anti-Western and repressive organization, in 2014 notoriously declared a self-styled caliphate that seized parts of Syria and Iraq, until the U.S. military intervened to help roll it back. The group's Syria and Iraq branch no longer controls any territory and is not known to be playing a role in the current fighting. But it's still a lethal force operating through sleeper cells in the two countries.

Ford said the fighting in Aleppo would become more broadly destabilizing if it drew Russia and Turkey — each with its own interests to protect in Syria — into direct heavy fighting against each other.

What do we know about the group leading the offensive on Aleppo?

The U.S. and U.N. have long designated the opposition force leading the attack at Aleppo — Hayat Tahrir al-Sham, known by its initials HTS — as a terrorist organization.

Abu Mohammed al-Golani emerged as the leader of al-Qaida's Syria branch in 2011, in the first months of Syria's war. It was an unwelcome intervention to many in Syria's opposition, who hoped to keep the fight against Assad's rule untainted by violent extremism.

Golani and his group early on claimed responsibility for deadly bombings, pledged to attack Western forces, confiscated property from religious minorities and sent religious police to enforce modest dress by women.

Golani and HTS have sought to remake themselves in recent years, focusing on promoting civilian government in their territory as well as military action, researcher Aaron Zelin noted. His group broke ties with al-Qaida in 2016. Golani cracked down on some extremist groups in his territory, and increasingly portrays himself as a protector of other religions. That includes last year allowing the first Christian Mass in the city of Idlib in years.

By 2018, the Trump administration acknowledged it was no longer directly targeting Golani, Zelin said. But HTS has allowed some wanted armed groups to continue to operate on its territory, and shot at U.S. special forces at least as recently as 2022, he said.

What's the history of Aleppo in the war?

At the crossroads of trade routes and empires for thousands of years, Aleppo with its bustling neighborhoods and ancient landmarks in the 2010s saw some of the fiercest fighting of Syria's civil war.

Aleppo was home to 2.3 million people before the conflict. Rebels seized the east side of the city in 2012, and it became the proudest symbol of the advance of armed opposition factions.

In 2016, government forces backed by Russian airstrikes laid siege to the city. Russian shells, missiles and crude barrel bombs — fuel canisters or other containers loaded with explosives and metal — methodically leveled neighborhoods. Starving and under siege, rebels surrendered Aleppo that year.

The Russian military's entry was the turning point in the war, allowing Assad to stay on in the territory he held.

This year, Israeli airstrikes in Aleppo have hit Hezbollah weapons depots and Syrian forces, among other targets, according to an independent monitoring group. Israel rarely acknowledges strikes at Aleppo and other government-held areas of Syria.