Syrian opposition forces have taken control of much of the country’s second-largest city Aleppo after a lightning advance that killed dozens of government soldiers in a major challenge to President Bashar al-Assad.

The capture of Aleppo by rebels, Syria’s second-largest city, has been a big change in the more than decade-long conflict, as the city had never before fallen out of government hands

The surprise offensive on Aleppo by Syrian rebel groups has left the US in the difficult position of not fully supporting either side while also maintaining a force posture of nearly 1,000 troops in Syria as part of the ongoing mission to fight ISIS.