did you ally all the italian factions prior to all this going off? My diplomatic policies ( in every game bar the first as english which was of mixed success because I practiced a great deal of catholic killing) are based around allying everyone and let them do the aggreement breaking. It makes them appear dishonorable and hence potentially gives them more problems because the other factions should trust them less.
Until in theory all aggresor factions become inherently venal and their word isn't worth a damn and they just get caught up in an endless cycle of war with each other.
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