STORY: Syrian rebels' capture of Aleppo from President Bashar al-Assad has brought the Syrian civil war back into focus, jolting frontlines that were dormant for years.

Assad's government has vowed to fight back and Russia has launched air strikes in support.

H.A. Hellyer of the Royal United Services Institute:

"I think he will try to crush the rebel advance in whatever way that he can. But the question will be whether or not he comes to the conclusion that he won't be able to, hence he's got to engage in some sort of political process."

:: Who are the rebels?

So who is leading the rebel advance?

A group called Hayat Tahrir al-Sham or HTS launched the campaign.

:: 2016

Led by Abu Mohammed al-Golani, HTS was formerly called the Nusra Front, and was al Qaeda's official wing in the Syrian war until it broke ties in 2016.

It has long been dominant in Idlib, part of the northwest that stayed under rebel control for the past few years.

And it's designated a terrorist group by the U.S., Russia and Turkey.

Another rebel alliance has launched a separate offensive from areas north of Aleppo.

They're backed by Turkey and fight under the banner of an armed opposition group called the Syrian National Army.

:: Why now?

So why has the conflict flared now?

Two supporters of the regime - Iran and Russia - are tied up with other conflicts, regionally and in Ukraine.

Iran-backed Hezbollah, in particular - which helped Assad's forces defeat the rebels in the past - suffered major blows in its conflict with Israel over the past few months, now halted by a ceasefire.

Speaking in Turkey, the head of Syria's main opposition abroad, Hadi al-Bahra, said the preparations to seize Aleppo began a year ago.

But were delayed by the wars between Israel and Hamas in Gaza and Hezbollah in Lebanon.

"They saw it is not good to have it while there is a war in Lebanon so the moment there were a ceasefire in Lebanon, they found that the opportunity is good for them to start."

Syria is partitioned into zones with foreign troops on the ground - including the United States and Turkey.

A deal between Russia and Turkey has stabilized the situation in the northwest since 2020.

Turkey, whose troops are in the northwest, has also expressed growing frustration with Assad's failure to reach a deal with the opposition to end the conflict.