The one thing you can do to keep your neighboring rival's busy is to build the teahouse right away and send shinobi to the ronin province that your rival just conquered.In the early part of the game everyone's armies are small,so it doesn't take more than one or two shinobi's to start rebellions that your rival has to waste time and koku to put down.

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Diplomacy is the art of telling someone to go to hell so that they look forward to making the trip.