Quote Originally Posted by dumas

-If that faction's army is in your land, you can attack that army without any repercussion.
I think the original post said that he did get in dutch with the Pope when he did this. My experience is that it's only safe to attack after you get the excomm warning if you're actually besieged.

Most of the time, if you can take it, let any Catholic enemy attack you, over and over, without attacking them. It wears them down, and drops their papal standing (since the AI isn't too good about bribing His Holiness) to the point you can safely assault. At that point, the enemy is weakened substantially and cannot resist after the Pope gives the All Clear to Kill sign.