• Rebels toppled the Assad regime in Syria after a brutal 13-year civil war.
• Once again, the region's power dynamics have been dramatically reshaped.
• And there are risks and opportunities for the US.

In a lightning two-week campaign that shocked the world, Syrian rebels led by the Islamist Hayat Tahrir Al-Sham group deposed Bashar Assad, the longtime ruler of Syria.

President Joe Biden cautiously welcomed Assad's removal on Sunday, calling it a "moment of historic opportunity."

With Assad gone, there is a lot at stake for the US, and analysts warn that it must work carefully to further its goals in the region, and avoid the country collapsing into chaos.

Burcu Ozcelik a senior research fellow for Middle East Security at the UK's Royal United Services Institute think tank, said Assad's defeat presents the US with an opportunity to further its longtime goal of denting Iranian regional power.

"With the overthrow of Assad, Iran has been dealt a strategic blow, meeting overarching US objectives to diminish and dismantle Iran's so-called axis of resistance," she said.

But Chris Doyle, director of the Council for Arab British Understanding, said that since the Arab Spring of 2011, the US has had no diplomatic relations with Syria, and its role in the country in recent years has largely been limited to defeating ISIS.

This means it is still figuring out how to handle the situation.

"All of a sudden, I think they're dusting down documents," he said of the US.

A divided country

When the Syrian civil war broke out in 2011, it quickly became a brutal struggle for power between rebel militias, government forces, and their powerful foreign backers, including the US, Turkey, Iran, and Russia.

The US provided training and support for some rebel groups, notably Kurdish militias and moderate groups, but stopped short of direct involvement in the conflict.

When Islamic State militia seized swaths of northeastern Syria in 2014, and used it as a base for terror attacks in the West, the US led an international campaign to destroy the group.

But it's played a largely limited role in the country since, and has around 900 troops in the northeast, whose task is to quash IS operations and defend the US' Kurdish allies.

This could limit its ability to play a larger role now.

Developing ties

Andreas Krieg, a Gulf specialist at the Institute of Middle Eastern Studies at King's College London, told BI that the US will likely limit its role to a "low-level campaign" fighting ISIS until a new authority is in place in Syria.

One key goal for the US will be to help restore order, defend its allies, and prevent another brutal power struggle among rival militias and religious groups that could spill over into neighboring countries.

According to Mohammed Albasha, founder of Basha Report, a Virginia-based consultancy specializing in Middle East affairs, the end of Assad's rule has led to a "significant security vacuum" that extremist groups could exploit to regroup and expand.

Against that backdrop, the "caretaker government may struggle to deliver basic services," worsening the already dire humanitarian situation, he said.

On top of that, he said the US will likely have to contend with Iran and Russia as they seek to rebuild influence. Both countries will likely act "swiftly" to strike favorable deals with emerging power brokers in Syria, he said.

"Without effective coordination, this could spiral into another civil war."

Iran's power damaged

For years, Iran helped prop up the Assad regime, providing vital economic and military support during the civil war.

With Assad gone, Iran's "Axis of resistance" of states and militias whose mission is to eradicate US regional influence and destroy Israel looks much weaker, according to the Royal United Services Institute's Ozcelik.

She added: "As the dust settles, Iran will seek to carve out a reformed role for itself in a post-Assad Syria in the months and years to come, but for now, Tehran is weakened in its foreign policy adventurism and reputation in the Middle East."

Russia has also suffered a defeat with the overthrow of Assad.

Russian forces played a key role in rolling back advances by rebels when Russia entered the conflict in 2015. According to reports, Assad and his family have taken refuge in Moscow.

With Assad gone, Russia may have lost access to strategically vital military bases in Syria.

Who will take over?

Speculation is swirling on who will take power in the vacuum left by Assad's deposal.

Among the key contenders is Abu Mohammad al-Jolani, leader of the Hayat Tahrir al-Sham militia that played a central role in defeating Assad, and who fought alongside an Al Qaeda affiliate during the US occupation of Iraq.

The HTS — a militant group designated as a terrorist organization by the US and the United Nations — has controlled Syria's northwestern Idlib Province, where analysts say it worked to consolidate power and transform its image while pursuing its ultimate goal of toppling Assad.

But the group's roots as an affiliate of the terrorist group al Qaeda will likely be of concern in Washington, which has a $10 million bounty on the head of al-Jolani that US officials are now reportedly discussing removing.

"The terrorist designation of HTS and al-Jolani's own violent legacy in Iraq against American troops makes him far from an ideal partner for peace from the perspective of Western policymakers," said Ozcelik.

In a post on Sunday, President-elect Donald Trump said the US should stay out of the conflict.

Edmund Fitton-Brown, a senior advisor to the Counter Extremism Project, an international organization formed to combat the threat from extremist ideologies, said the main US concerns revolve around whether the HTS would seek stable governance, or continued insurgency.

"Some aspects of their rule in Idlib have been exclusionary and tyrannical," he said, "yet they claim to have cut ties with Al-Qaeda and to embrace diversity (Christians, Kurds, etc.) as part of Syria's identity."

Yaniv Voller, a senior lecturer in Middle East Politics at the University of Kent, meanwhile, said he struggles to see how Washington could work with al-Jolani directly unless he completely abandons his jihadist rhetoric and animosity toward Israel.

"Jolani is associated with al Qaeda and throughout much of his 'career' has expressed staunch anti-American and anti-Western views," he said.

However, he said another risk is that Syria breaks into territories controlled by competing militias and warlords, which he said would turn Syria into a potential base for terrorist activities.

From a US perspective, that would arguably be far worse.