What's happening in Syria? What to know about the complex, long-simmering civil war

A surprise offensive by rebel forces opposing Syrian President Bashar al-Assad's regime has led to a new round of fighting in the Middle Eastern country's long-running – but until recently largely static – civil war.

After a lightning advance in just a few days, rebels captured almost all of Aleppo, which is Syria's second-largest city. However, Assad in a speech Saturday vowed to defend the country's "stability and territorial integrity."

Syria's civil war grew out of Assad's brutal crackdown on pro-democracy protests in 2011. More than a decade later, about half a million people have been killed, according to The Syrian Observatory for Human Rights, a U.K.-based monitoring group. Millions of Syrians have fled overseas. Key infrastructure lay in ruins. A web of Syria's neighbors and countries around the world have backed either Assad's government or various rebel opposition groups to varying degrees.

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Here's what to know about the complex war.

Why has Syria's civil war re-erupted now?

A cease-fire brokered by Turkey, which supports the opposition, and Russia, which backs Assad, has been in place since 2020. But the fresh rebel offensive started last week as anti-Assad fighters sought to take more government-controlled territory. Rebels claimed this was in response to an uptick in attacks from Assad's forces.

However, analysts say the eruption of new fighting is likely a direct consequence of what's been taking place outside Syria's borders. That includes Iran-backed Hezbollah – an ally of Assad – and its major hostilities with Israel in Lebanon. Russia's war in Ukraine has also influenced the region.

"Hezbollah has been weakened by its recent conflict with Israel. Russia remains entangled in the war in Ukraine and faces economic strain. Iran is stretched thin, with its ground lines of communication and regional allies under direct attack," said Mohammed Albasha, founder of Basha Report, a Virginia-based consultancy specializing in Middle East affairs. "Against this backdrop, Turkey and its allies are seizing the moment."

Who's fighting who? And why?

A Sunni Islamist militant and political group named Hayat Tahrir al-Sham, or HTS, and its allies have been leading the latest rebel offensive. HTS has historic, though now splintered, connections to both al-Qaida and the Islamic State terror groups. HTS wants to topple Assad, expel Iran from the area and pursue its own extremist ideology.

Assad's chief military supporters have been Russia and Iran. Russia entered the conflict in 2015 to shore up Assad's struggling regime. Russia's leader Vladimir Putin wanted to assert his country's power on the world stage and act as a counterweight to the U.S., which at the time was arming and providing air cover for a motley crew of anti-Assad rebel groups, including Kurds in Iraq and Syria, who were simultaneously waging war against the Islamic State.

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Iran has long backed the Assad regime, funneling money and fighters to it through Hezbollah. It does this to pursue its national security aims, which include using Syria to pressure its neighbor Israel, Iran's arch-enemy.

Turkey, Syria's northern neighbor, has thrown its weight behind anti-Assad opposition groups because it wants to contain Kurdish secessionist movements in the region, including in Turkey, where separatists have for decades waged what the government describes as terror attacks. Turkey's President Recep Tayyip Erdogan also faces growing domestic political pressure to deport Syrian refugees who have fled to Turkey in droves since the war's start.

What does the fighting mean, and what happens now?

Assad's regime is entrenched. His government as of last week controlled approximately 70% of Syrian territory, according to the United States Institute of Peace, a think tank. The surge in fighting from a mostly dormant cease-fire raises the possibility of yet another violent and volatile front in the Middle East after a fragile cease-fire in Lebanon between Hezbollah and Israel and the ongoing fight in Gaza between Iran-backed Hamas and Israel.

Whether Assad's government can survive, and prosper, may ultimately depend on how much intervention there is from Russia, Iran and Hezbollah, all three of which have enabled him to stay in power.

Syrian rebels sweep into Aleppo: army says dozens of soldiers killed

Ayman Abdel Nour, a former friend of Syria's leader from their college days studying medicine in Damascus and the editor-in-chief of All4Syria, a leading independent news outlet, said that in recent weeks Assad has resisted efforts by Russia to get him to meet with Turkey's Erdogan ahead of President-elect Donald Trump's second term.

This meeting, Nour said, would be aimed at finding a longer-term solution to Syria's 13-year-old simmering civil war – in particular its underpinning by non-state actors such as Hezbollah at a time when Trump has signaled he wants to withdraw the remaining 900 U.S. troop in Syria who counter Iran's influence.

"The rapid gains by rebel forces, combined with the maneuvering of external powers with conflicting interests, leave Syria's future precariously uncertain," said Albasha.

This article originally appeared on USA TODAY: What's happening in Syria? What to know about its complex civil war