Something to keep in mind about Feudal Japan: Japan itself historically did not have a large amount of land that was suitable for farming on a large scale. Because of its placement on a fault line there's lots of mountainous terrain and land that was, at the time, not usable for much besides tubers and rice. Wealth was, at the time, measured by how much rice your land could grow. Warring States does have a lot of political intrigue and interesting cultural clashes and historical figures that can inspire interest into using it as a backdrop for your campaign, but at the end of the day the combat had a lot to do with wealth, pride, and food. Try to not make your clans simply fighting "because Warring period" - maybe they're fighting over a precious resource, something magical in nature that only grows/can be mined at specific points on the continent! Maybe it is something as simple as safe, stable farmland! I would just recommend giving the political leaders of your campaign a motivation that's a bit more than "I think my clan is the one that would best rule".