That 2016 bombing of the Syrian Arab Army in the Thardeh Mountains.

A detail about that incident:

I have personally met a Syrian soldier who survived the [September 17, 2016] US coalition bombing in Deir-ez-Zour. He told Syrian Formal TV that US drones were flying over the airbase for more than four and a half hours before the US aircraft made their “mistake”. He said that he originally thought they were scanning the area to help the fight against ISIS. However he gradually realised that the drones were actually undertaking surveillance of the base itself – of its equipment, tanks, ammunition, etc. Later, the US aircraft destroyed all this whilst the ISIS terrorists were screaming Allah Akbar!^[1]

Prof. Tim Anderson has analyzed that attack in detail.^[2]

That extensive prior surveillance aspect rings a bell somehow. It’ll come to me, I’m sure.

Notes
^[1] "Here’s How the US, Israel, al-Qaeda and ISIS Work Together in Syria." By Afraa Dagher, Mondialisation, 9/21/16.
^[2] "Implausible Denials: The Crime at Jabal al Tharda. US-led Air Raid on Behalf of ISIS-Daesh Against Syrian Forces ." By Prof. Tim Anderson, Global Research, 12/17/17.