[Kanan is going to study his feet very carefully in this. Bringing up these memories after all these years is more painful than he expected, and he'd never dream he'd have to actually explain it to those who were caught up in the middle of it all]

The Emperor, and the clone army.

I...spoke, briefly, to a couple of clones here and I'm not so sure how voluntary their betrayal was, but it was a plot from the beginning by the Sith to rid the galaxy of Jedi.