Thanks every one, you've really given me a much better understanding of this part of the game - not really explained in the manual.

Generally I've been bribing or fighting rebel armies, but it is a bit of a chore, and I'd been wondering whether the effort was worth it. Clearly it is.

Is there any way of stopping rebels appearing in the first place? What is the trigger for their appearance? Perhaps the level of happiness in my cities?

Would forts or my own armies at my borders stop the rebels from entering my provinces, or do they spontaneously generate within my borders?

Most of my battles at the moment are against piffling rebel armies.