Listen, I know that not everyone has read Asunder, so consider this a psa. Also, it’s a bit spoilery, so read with caution.
Anders didn’t start the mage/templar war. And neither did Fiona.
The person who started the mage/templar war was Lord Seeker Lambert.
It’s well known that the mages were holding a conclave. Under Chantry law they have every right to do so, though of course being prisoners in the Circles makes them dependent on the good will of the Divine and the Templars, so actually holding a conclave is easier said than done.
Inquisition gives off the impression that the mages held their conclave and cast their vote, and then the war began.
That isn’t…actually what happened.
The mages had their conclave, they were fighting/discussing among themselves. Fiona famously says “Fuck the Divine”, and while all of this is going on, Templars have been leaving the room, supposedly to inform Lord Seeker Lambert of what was happening.
Fiona puts forth the motion to seperate, and Lord Seeker Lambert storms in and declares treason.
And then the Templars slaughter the mages.
What caused the mage rebellion, in the end, was exactly what Anders had been fighting to bring to light: violent oppression, and the slaughter of innocents.