"Aleppo wouldn't rise even if it took Viagra." So went the joke among many of the Syrian rebels and civilians I met when last in the region a few months ago.

As the forces of President Bashar al Assad's regime blitzed Daraa, Homs and Damascus, there was general bewilderment and dismay among Syrian revolutionaries that the residents of the country's largest city and commercial hub had shown little appetite for dissent, let alone armed insurrection against the regime.

Yes, there were a handful of demonstrations on the campus of Aleppo university and a few scattered Friday protests, but nothing to match the out-and-out street fighting going on elsewhere. As the months passed, what began as great anticipation of an Aleppo uprising slowly descended into frustration over the city's comparative silence.

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In the space of a few days however, all that has changed.

As I write this, fierce clashes rage across Aleppo. Yesterday government forces laid down an intensive artillery and mortar bombardment, notably in the Salaheddine and Mashhad districts, as both rebel and regime forces hurriedly tried to reinforce their frontline fighters.

Thousand of families are in turn fleeing and neighbourhoods making desperate appeals for blood supplies.

Aleppo is bleeding now for the revolution and is likely to do so in a way unprecedented to date in Syria's bitter civil war.

The stakes in the battle for this ancient and beautiful city that stands as a United Nations World Heritage site are as high as they get, and neither side can afford to back off.

Just why though, did it take so long for the full-blown version of Syria's revolution to erupt here, and what does Aleppo's involvement now tell us about the direction the conflict might be going and its likely outcome?

It's fair to say that the answers to both parts of this question are inextricably connected.

To begin with, it's important to realise that Aleppo, by and large, has benefited economically under Mr Assad's rule. While the regime remained stable, so the city and its inhabitants prospered.

While Aleppo's wealthier citizens have always had ties to Mr Assad's government, even the city's underclass has done fairly well from the regime. How ironic it is that the war next door in Iraq contributed so much to this prosperity. According to one businessman, quoted in the influential magazine, Foreign Policy, several traders from Aleppo received payments on million-dollar contracts they never had to fulfil after the demise of Saddam Hussein. An influx of Iraqi refugees also helped give rise to an increase in rent and land prices that saw some houses in poor Aleppo neighbourhoods almost tripling in value.

How strange it is to think that today that human tide of refugees flows in the opposite direction, as many Aleppans now flee to the comparative sanctuary that is Iraq.

Another factor too that made for Aleppo's seemingly lethargic rise to the revolutionary cause is it sheer size and diversity.

With a population of more than two million, comprising a Christian minority as well as Kurds and Turkmens, Aleppo's rebel activists were faced with immense challenges in organising across the city and were vulnerable to penetration by Mr Assad's intelligence and security services. Time and again as the revolution's local co-ordination committees tried to mesh into an effective network they were infiltrated and neutered.

Mr Assad's regime simply cannot afford to lose Aleppo, standing as it does barely 40 miles from the border with Turkey.

Knowing this, it unleashed subversive levels of violence, shocking even by Syria's current standards, yet subtle and far removed from the open fighting and indiscriminate bombardment the city's inhabitants are now experiencing.

Massive numbers of thuggish loyalist militiamen, known in Arabic as "shabiha" or ghosts, were deployed into the city.

While in other restive areas these shabiha were drawn from Assad's Alawite sect in Syria's coastal regions, those utilised in Aleppo are overwhelmingly Sunni and from the surrounding Arab tribes.

According to one anonymous Syrian activist and blogger who goes by the name Edward Dark, one tribe, the al Birri clan, acts as Mr Assad's primary enforcer in the city.

"They don't even try to hide it and openly boast of receiving weapons and arms from the regime," he says.

"There are lots of others too, usually convicted criminals involved in various smuggling or drugs. They were offered pardons and funds in order to help the security forces in the crackdown."

For now, inside the city the rebels are preparing for the worst. Yesterday columns of regime troops and tanks were moving in convoy from Hama and from the border posts with Turkey in Idlib province

No doubt, once the Syrian army and airforce feel they have sufficiently pulverised Aleppo's restless neighbourhoods into submission, those soldiers and tanks will move in to begin a house-to-house "cleansing" operation, as they did in other places such as Homs, resulting in terrible atrocities and a high number of casualties.

Some observers say Mr Assad probably realises he is finished and is now simply fighting for time in an effort to create an Alawite rump state and bastion, that he and his surviving loyalist henchmen can cling to on the Mediterranean coast.

That may well be the case, but for now he shows no signs of giving up Aleppo, and it is clear the regime's core is willing to engage in any amount of destruction in its desperate attempt to hang on. That said, by bombing Aleppo, a city once relatively quiescent, the regime is only further digging its own grave.

Yesterday France insisted that Russia and China must act within the United Nations Security Council to stop a possible bloodbath in Aleppo.

No-one, it seems, is listening – least of all those on both sides of the frontline divide digging in around the city.

Make no mistake about it: the battle for Aleppo will be nothing less than a do-or-die battle for Syria's revolution and the survival of Bashar al Assad.