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[Forward Observer]

Remember that whether a clan is Shogunate or Empire, all are nothing more than petty warlords who are basically looking out for #1. If it either bothers you to turn on an ally, or if it is the fact that said ally may presently be too strong to attack and/or also allied with others even stronger, then simply incite a revolt in their province closest to you. Chances are the rebels will take it, and then you can move in without declaring war. You can eliminate whole clans this way without ever being at war with them.

The downside is that sometimes the rebel army that spawns will be large, they will hole up in the castle, and after defeating them you will find everything quite damaged--which will present you with a substantial repair bill. However, it's still usually cheaper than attempting to build up an extra stack of troops and ships to chance a invasion of a distant enemy province--especially early in the game when your economy is just getting off the ground.

Cheers

P.S. If any of the damaged city structures don't fit your needs, don't hesitate to torch them instead of repairing. Then you can build to suit your particular gameplan on the next turn.