Value of excommunication and alliances?

[DensterNY]

I'm playing out my first full campaign (on HARD) so its mostly inexperience that leads me to this but something happened that makes me question the value of excommunication and alliances.

As the English I eliminated the French faction with help from my German allies who got excommunicated because they attacked the French first. Everybody ended their alliances with the Germans except for me and I expected a long friendship between our peoples and I even married two daughters off to them.

Only the Hungarians attacked the Germans so my allies moved their offensive forces to the East and I moved my forces to the South to conquer the Aragonese. I kept decent though modest defensive armies bordering the Germans... after a few turns I ended the Aragonese and then suddendly **WHAM** The Germans attack me... even though they were fighting for several provinces with the Hungarians.

So it leaves me wondering... What threat is there of excommunication if other Catholics will only attack if it suits their interest? Also, outside of trade and an implied agreement that you're not likely to attack what value is there in alliances?

- This whole thing got me thinking screw the pope and screw allies, as long as I have my nation well tuned to my warmongering and my vast armies keep me as the proverbial 800 pound gorilla then I ANSWER TO NO ONE... War will be my religion and my sword will be my friend.