Syrian and Russian jets are stepping up strikes on opposition forces in northern Syria in retaliation for the sudden offensive that has cost the autocratic regime control of the country’s second largest city,

In just a few days, a patchwork of rebel forces has seized control of a broad stretch of land in the west and northwest of Syria. But it’s unclear just how much support Syria’s weakened allies can provide.

Hayat Tahrir al-Sham (HTS) is a Sunni Islamist political and militant group that was set up in 2011 which has a key role in the Syrian civil war. The leader of the Islamist Group (IS) Abu Bakr al-Baghdadi was involved in the formation of HTS in 2011 under a different name, Jabhat al-Nusra.