This seems like a crime against the Church, since the Pope is the highest of God's ambassadors. How was such a Catholic nation willing to invade the center of their religion?
Found this thread that might answer your question:
https://www.reddit.com/r/AskHistorians/comments/5uscno/how_did_italian_catholics_justify_wars_against/.
where /u/AlviseFalier answers your question.