For the past few years, Syria has been locked in an uneasy stalemate, fragmented and devastated after more than a decade of civil war, but with the front lines frozen and the worst of the fighting seemingly over.

The regime of President Bashar al-Assad had claimed a Pyrrhic victory after brutally crushing a rebellion with the military backing of Russia, Iran and Iranian-backed militants. It had regained control of most of the war-ravaged country, while the remnants of the armed rebellion were pushed back to enclaves in the north and north-west, surviving under the patronage and protection of Turkey.

But this week, that fragile impasse was shattered as insurgents led by the Islamist group Hayat Tahrir al-Sham advanced from its stronghold in Idlib province and launched a lightning raid across the north against regime forces, charging into Aleppo, Syria’s second-largest city. By Saturday, they were closing in on Hama, to the south.