JOHN YANG: Good evening, I'm John Yang.

For more than a half century, Syria had been ruled with an iron fist by the Assad regime.

First Hafez Al Assad, then his son Bashar.

It all came to an end this weekend with stunning lightning speed as rebels moved into the Syrian capital of Damascus, took the city, and with it, the country.

JOHN YANG (voice-over): Across Syria today, gunfire, chanting in cheers in celebration of the end of an era.

The rebels faced no opposition as they advanced overnight, Russian state media said President Bashar al-Assad fled to Moscow where he was granted asylum.

At a mosque in the capital city, the leader of the largest insurgent group addressed his fighters.

ABU MOHAMMED AL-GOLANI, Syrian Rebel Commander (through translator): This victory, my brothers, is for the Islamic nation.

This victory is a new history for the region.

JOHN YANG (voice-over): Ordinary citizens roamed the presidential palace in Damascus, rifling rooms and taking selfies.

Across the country, symbols of the Assad family's repressive 53 years in power came toppling down.

In 1974, President Nixon became the first U.S. president to visit Syria, meeting with Hafez al-Assad, who had seized power in a coup.

Hafez and son Bashar ruled Syria by relying heavily on security forces to crush dissent.

And they forged alliances with Russia and Iran.

A civil war ignited in 2011 killed hundreds of thousands and forced millions of Syrians to flee their homeland.

Today, the Syrian diaspora celebrated in Erbil, Iraq, in Istanbul, in Paris.

HARIRI LAMA (through translator): A renewal, we are starting on new foundations without all the tyranny.

JOHN YANG (voice-over): And in Berlin.

MAN (through translator): This day to day last night has the same meaning as the fall of the Berlin Wall.

This is freedom.

JOHN YANG (voice-over): The speed of Assad's collapse surprised many around the world.

At the White House today, President Biden spoke of the days ahead.

JOE BIDEN, U.S. President: The fall of the regime is a fundamental act of justice.

It's a moment of historic opportunity for the long suffering people of Syria to build a better future for the proud country.

It's also a moment of risk and uncertainty.

JOHN YANG (voice-over): The rebel supporters wasted no time in unfurling the opposition flag at Syrian embassies around the world.

JOHN YANG: Even as these events unfolded, U.S. Central Command said it carried out airstrikes against known ISIS camps in Syria, hitting more than 75 targets.

What lies ahead for Syria and the Middle East is an open question.

Hassan Hassan is the founder and editor-in-chief of New Lines magazine.

So, a regime that controlled Syria for more than half a century has been erased in what feels like a week and a half.

How big a deal is this?

What's the significance of this?

HASSAN HASSAN, Founder and Editor-in-Chief of New Lines Magazine: It's a big deal, not just for Syria, for the entire region.

The regime has been slow kind of disintegration over the past decade or so.

It needed to be saved twice.

First by Iran in 2013, when the regime looked like it was losing very fast major parts of the country.

And then later in 2015 by Russia, when also the regime started to feel under so much pressure by the rebels, you know, Iran failing to save it the second time.

JOHN YANG: What do we know about the main insurgent group, Hay'at Tahrir al-Sham, which the State Department classifies as a terrorist group, and the leader of that group, Abu Mohammed al-Golani?

HASSAN HASSAN: We know quite a bit of it.

It's been a prominent part of the Syrian rebel groups over the past.

Like I said, over the course of the conflict they started.

They evolved over time, I think like at least three, four times over the course of the conflict.

First started as part of ISIS in Iraq to just keep it simple, and then left ISIS and then joined al-Qaeda and then still left al-Qaeda and clashed with both, and then evolved from a kind of a jihadist dominated force into still a jihadist organization, but away from global jihad into kind of a Syria focused organization.

It's been evolving since.

JOHN YANG: Is there any hint or sense of what government or what's going to be next for Syria or is there a power vacuum that could cause problems?

HASSAN HASSAN: So a lot of people kind of think, is this another Iraq chaos and then civil wars and so on and so forth kind of a long saga of kind of just violence.

Or Libya and other places also face the same thing.

I think to me, Syria start not from stage one of the Iraq War, but advanced stages when, you know, after civil wars, after exhaustion, people start to kind of try to work together and try to find something.

Remember the Syrian rebels who just taken over Syria have anywhere from five to 10 years of governing in Syria, and they've gone through all the civil wars, fighting each other and fighting until they succeeded in the past few years to unify their ranks, become more organized, become better at governing and working together through coalitions, getting training as well.

They've been preparing for this moment for many years as the Assad regime has been weakening, especially over the past week -- past year, when the Assad regime, other Iranian allies in the region have been entered this relentless campaign by Israel.

Hezbollah in Lebanon.

Hezbollah has been key ally of the Assad regime.

Without Hezbollah, without Iran, the Assad regime cannot contain and continue sustain, unsustain fighting in Syria.

That's why when Hezbollah has been weakened, when Iranian -- the Iranians have become -- it's become harder for them to freely operate in Syria.

That's when the Assad regime has become so exposed to such an attack.

Then obviously, don't forget that Russia has been also entangled in the Ukrainian war.

So Assad is left without allies and the rebels are filling the gap.

JOHN YANG: You mentioned that this is a big deal not just for Syria, but for the region.

Talk about that.

What has this done to the balance of power in the region?

HASSAN HASSAN: I think the best way to think about it is Syria is a piece in the puzzle, in the Iranian in the region, but also as part of the Iranian axis.

And like I said, Israel, the fall of Assad has to be seen in the context of Hezbollah just being finished essentially in Lebanon, or weakened significantly, severely reduced to kind of a small, you know, faction as opposed to the previous power that the jewel of Iranian access in the region.

So it's a kind of two in a row.

Hezbollah has been weakened by Israel.

A day, in fact, after Hezbollah had to sign what looked like it's a kind of a surrender deal with Israel, the Syrian rebels launched this operation.

So now you have two of the most important allies of Iran in the region essentially either weakened or gone.

And Iran is left with almost no influence or very little influence in the Levant, Gaza.

Hamas has been weakened.

Hezbollah in Lebanon has been weakened, and Assad is out of the picture.

So now Iran is left with Iraq, influence in Iraq.

And there are -- there's chatter that Iraq is going to be next in terms of pressure by Iranian like Israel and others who will take on Iran -- in Iraq and obviously Yemen with the Houthis.

JOHN YANG: Hassan Hassan, thank you very much.

HASSAN HASSAN: You're welcome.

Thank you.