The Battle of Aleppo seems to be shaping up into one of those key moments that could determine the outcome of what we can probably now call the Syrian civil war.
It is Syria's second largest city and its commercial hub, classified by UNESCO as a world heritage site because of its priceless cultural and architectural treasures. Until relatively recently, it had been able to stand aside from the turmoil that was tearing the rest of the country apart.
Not any more. A week after battles raged in parts of the capital, Damascus, now it's Aleppo's turn, as the rebels ratchet up the pressure in the apparent belief that their moment has come.
So where does that leave Syria's neighbours, and where does it leave the rest of the world? I was struck by a line in an impressively dispassionate report published by the defence policy think-tank the Royal United Services Institute: " ... the question of some sort of Western intervention in Syria has shifted from a predilection to stay out of the conflict in any physical sense to an awareness that intervention is looking increasingly likely. We are not moving towards intervention but intervention is certainly moving towards us."
The argument goes like this: increasingly foreign governments are worrying about what's likely to happen when (not if) Bashar al-Assad is forced out. In the words of RUSI's director, Professor Michael Clarke: "... the regional implications of this dynamic of violence are more a driver of international diplomacy than the human misery inside Syria itself."
Syria's neighbour Lebanon is a tinderbox where just one spark can ignite a conflagration; and Iraq and Iran, with their Shia majorities, are lined up against Turkey and Saudi Arabia, with their Sunni majorities. Professor Clarke talks of an "arc of proxy confrontation" between Iran and Saudi Arabia, which could lead to all kinds of trouble.
Perhaps the question should be not "Will outside powers intervene in Syria?", but "Will they intervene more than they are already?"
As the RUSI report points out: " Already, it is believed that western intelligence and special force operations are actively underway ... Western countries have backed the growing supply of arms, via Arab sources, to rebel forces for some months now ... Saudi Arabia and Qatar, with US and Turkish facilitation, have been arming and funding the opposition; and this covert support has been substantially responsible for the progress opposition forces have made in recent weeks."
On the government side, the report says: "Several Russian ships carrying a range of military equipment for the Assad regime are already at sea ..." And the Iranians are unlikely to stand idly by if they see their strategic interests put at risk: in order to preserve their influence with Shia groups in Lebanon, for example: "the Iranians would provide weapons, materiel and probably elements of the Islamic Revolutionary Guard Corps to support Assad, as they did successfully with the Shia militias in Iraq."
No one is suggesting that the West is likely to mount an Iraq-style international ground invasion, nor even a Libya-style air campaign. But RUSI's Jonathan Eyal has this stark warning: "Western governments which have long worked for Assad's departure should now begin to fear what may lie in store. For, instead of imploding as other Arab countries did when they were gripped by revolutions, Syria will explode, disgorging its troubles across the entire Middle East, with potentially catastrophic consequences which will need to be managed, since they look unlikely to be avoided."
And he makes this additional point: "President Assad was [note the past tense] the region's last secular strongman, the last of the Arab leaders who repressed religious and ethnic differences in the name of a higher pan-Arab ideology. His method of government is now as defunct as that of the Soviet Union or communist Eastern Europe, on which it was based."
Until recently, the conventional wisdom was that international intervention in Syria simply wasn't on the cards. If the RUSI report is right, it may be time to rethink some of those assumptions: just watch the price of oil if tensions rise and Western governments start to panic at the thought of further price pressures as they struggle to save their faltering economies.