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If you set up a long-term regular tribute to The Papacy, like 20 for 20 for example, and create a few (3/ 4) Assassins where your Thieves Guild is, you can really play the Pope when attacking fellow Catholics. Specifically, after the tribute has started and you're ready to attack a Catholic nation, use Assassins, not for their conventional use, but to destroy all the religious buildings in your target factions lands.
This way their rep with the Pope will be rock-bottom, yours will be high, and old Popey will be far less inclined to intervene when you start wiping them out. (I noticed - but this may not always be true - that he warns you and your rep drops rather than him threatening excommunication as normal) It works
