First...the latin translates (approximately) to "It is sweet and glorious to die for one's country."
As for the rules of international war, etc I'm not sure how well established and/or standardized those are. Best bet if the guy joined an alliance is to peace out. If the other alliance then threatens to attack, they would be an aggressor, and if I were you I would hope you and your alliance have friends and treaties that will discourage such action.
Standardizing rules of engagement, etc would be a 'pipe dream'. There is little agreement on the virtues of raiding in the first place, then you get into the definition of an alliance, and at the end of the discussion very few points would get universal agreement.
Good luck.