Syria's devastating 13-year civil war was mostly dormant for years.
But a major offensive by militants in northwestern Syria has revived the conflict and dragged in regional powers, including Russia.
Since late November, Hayat Tahrir al-Sham (HTS) and its allies have seized control of most of Aleppo, Syria's second-largest city, and the surrounding countryside.
The offensive poses a major threat to President Bashar al-Assad, who has maintained his grip on power with the backing of key allies Russia and Iran.
The conflict has pitted Moscow and Tehran against Turkey, which supports armed groups involved in the HTS-led offensive.
Hayat Tahrir al-Sham
HTS is a militant Islamist group that seeks to establish a state in Syria governed by Islamic law. Many of its members are followers of Salafism, an ultraradical sect under Sunni Islam.
The group first appeared in 2012 under the name Jabhat al-Nusra, or the Nusra Front, the Syrian branch of Al-Qaeda.
Based in the northwestern province of Idlib, it later changed its name several times and distanced itself from Al-Qaeda. In 2017, it merged with other opposition groups to form HTS.
A U.S.-designated terrorist organization since 2018, the HTS has between 5,000 and 10,000 fighters, according to U.S. intelligence estimates.
HTS is allied with several smaller militant Islamist groups, including Faylaq al-Sham, Jaysh al-Izza, Harakat Ahrar al-Sham, and the Turkistan Brigade. The latter is made up of foreign fighters from China and Central Asia.
A militant tears down a portrait of Syrian President Bashar al-Assad in Aleppo on November 30, after jihadists and their allies entered the northern Syrian city.
Turkey-Backed Syrian National Army
Turkey, Syria's northern neighbor, is a major player in the conflict.
Ankara supports the Syrian National Army (SNA), a rebel group involved in the ongoing offensive in northern Syria. Turkey also has some influence over HTS, experts say.
The main target of the SNA has been the U.S.-backed and Kurdish-led Syrian Democratic Forces (SDF).
The SNA recently seized control of Tel Rifaat, a predominately Kurdish town in northern Syria. The Kurds are a long-oppressed ethnic minority in Arab-majority Syria.
Anti-government fighters patrol a street in the predominantly Kurdish town of Tal Rifaat on December 2, after pro-Turkey factions seized it.
Since 2016, Turkey has launched several cross-border ground operations against the SDF, a key Western ally in the campaign against the Islamic State (IS) extremist group.
The SDF is a coalition that includes the Kurdish People's Protection Units, or YPG. Turkey views the YPG as an extension of the Kurdistan Workers Party (PKK), which is designated as a terrorist group by Turkey and the United States.
The United States maintains around 900 troops in southeastern Syria along the border with Jordan and Iraq.
Iran
Iran has a longstanding relationship with Syria, where it has significantly boosted its influence since the civil war erupted in 2011.
Tehran intervened militarily in 2013 and played a key role in shoring up Assad's forces. It deployed hundreds of Islamic Revolutionary Guards Corps (IRGC) officers to recruit and train tens of thousands of local and foreign Shi'ite fighters.
For Iran, Syria provides a crucial land corridor to the Levant that is considered the logistical backbone of the so-called axis of resistance, Tehran's loose network of regional allies and proxies. Syria is the only other state actor in the axis.
Hamidreza Azizi, fellow at the German Institute for International and Security Affairs, said ensuring Assad stays in power is "even more important now for Iran than it was before."
That is because Tehran intends to use the land corridor through Syria to rearm and resupply its Lebanese ally Hezbollah, whose military capabilities were degraded following the recent war with Israel, said Azizi.
Iran has spent billions of dollars to keep Assad in power and is unlikely to ditch its investment, experts say.
Russia
Russia's military intervention in Syria's civil war in 2015 changed the tide of the war. Moscow's aerial campaigns against rebel positions helped the Syrian Army and pro-Iran militias regain swaths of territory but also caused widespread civilian casualties.
"When Russia first intervened in Syria's civil war, it was mainly about supporting an ally in the region, wanting to shore up an existing regime and avoiding instability that would follow its collapse, and wanting to push back against terrorism," said Jenny Mathers, senior lecturer on Russian politics at Aberystwyth University.
Russian warships sail during a naval drill in the Mediterranean Sea outside of Tartus on September 12.
But now it is also a matter of pride, Mathers said, because Moscow would "lose face" if the Assad government were to fall after years of Russian support.
Russia has two major military installations in Syria: an air base in Khmeimim and a naval base in Tartus, which provides Moscow access to the Mediterranean Sea.
Experts say losing an ally in Syria would be a blow to Russia's prestige on the international stage and its regional ambitions.
Since the HTS and its allies launched their surprise offensive, Russia has launched dozens of air strikes in areas under the militants' control.