Student or Learner
Deir Azour in the Kurdish north was a scene of violent clashes between security forces and defectors who had been urged to swap sides by thousands of chanting activists.
Have I understood this sentence?
There was a scene of clashes between security forces and defectors. The defectors had been urged to swap sides by thousands of activists before these clashes happened and they, apparently, as they are called defectors here, had swapped sides in the recent past, although how long ago it was we can't tell.
Your interpretation appears to be correct.