What does the Syrian Opposition offensive mean for Bashar Al-Assad?

The balance of power may be shifting in Syria years after President Bashar al-Assad regained control of some major cities in the northwest. In just a week, opposition forces have overrun Aleppo and Hama. And they say they have their sights set on Damascus. The assault is led by armed group Hayat Tahrir al Sham. It says it's responding to government attacks on its stronghold, Idlib. The Syrian military is reinforcing its lines of defence, and allies Iran, Hezbollah and Russia are promising to help. But can it push back the opposition advance? Is Assad's government really under threat? And what does this flare-up, after a period of relative calm, mean for millions of Syrian civilians?