Eastern Aleppo, the rebel-held half of what was once Syria's biggest city, is falling. Once the resistance there collapses, things may move very fast in Syria, and the biggest question will be: do the outside powers that have intervened in the war accept Bashar al-Assad's victory, or do they keep the war going?
Even one year ago, it seemed completely unrealistic to talk about an Assad victory. The Syrian government's army was decimated, demoralised and on the verge of collapse: every time the rebels attacked, it retreated.
There was even a serious possibility that Islamic State and the Nusra Front, the extreme Islamist groups that dominated the rebel forces, would sweep to victory in all of Syria. But then, just fourteen months ago, the Russian air force was sent in to save Assad's army from defeat.
It did more than that. It enabled the Syrian army, with help on the ground from Shia militias recruited from Lebanon, Afghanistan and Iraq and mostly trained and commanded by Iranian officers, to go onto the offensive. Assad's forces took back the historic city of Palmyra. They eliminated the last rebel-held foothold in the city of Homs. And last summer they began to cut eastern Aleppo's remaining links with the outside world.
In July government forces took control of the Castello Road, ending the flow of food and supplies for eastern Aleppo's ten thousand rebel fighters and its claimed civilian population of 250,000 people. (The real total of civilians left in the east of the city, once home to around a million people, is almost certainly a small fraction of that number.)