England attacks me, yet I'm threatened with Ex-com?

[Slug For A Butt]

The first thing I do at the start of any catholic campaign is to send a diplomat to kiss the Popes arse. Just leave the diplomat there to boost your popometer ratings with a gift of about 700 florins whenever the Pope forgets how wonderful you are.
Whenever I'm about to go to war with another catholic faction, defensive or offensive, I pop him a couple of florins to get my rating above the rating of my soon to be enemies. Otherwise he just seems to toss a coin, doesn't seem to matter who's being aggressive. When you get the Popes warning about excommunication, just ignore it, if you are on more favourable terms than your soon to be enemy it seems to have no discernable effect on his attitude to you.

In short... just keep throwing coins at him and he will let you get away with murder or even genocide, even when he warns you not to.
And lets be realistic, a couple of 700 florin gifts at the right time will make you a rich man (and with a little luck get your enemy excommunicated).

Good luck.