Ah, I see you've already cut the head off the dragon

I think bribery might be the best was forward in Treb - then send your new -Horde against Rome
... and that would be your next best move, so long as you have enough of an economy left. As Sun Tzu probably didn't say: "If you can't beat em, bribe em"

So long as you can afford the initial bribe it's always nice to have a stack or two of turncoat mongols in your armies (pretty cheap upkeep, at least in the original - I don't know if BKB's altered that, though). And the Pope deserves it, what with his superstitious "Prester John" fantasies