Since time immemorial, Nihon-Ja has been a country with large inequalities. The Nihon-Jans were divided between castes: the merchant-caste, the farmer-caste, the handcrafter-caste, the lumberjack-caste (Kikori) and the warrior-caste, the Senshi, which held most power. It's members were the only Nihon-Jans alowed to carry weapons in public, and the clans of Senshi were very powerfull.
The emperor of Nihon-Ja, Emperor Shigeru, was a liberal, which meant that he gave the other castes more and more rights, untill they were almost equal to the Senshi. The clans didn't like this, but when they reported their complaints to the emperor, he responded by saying that they were just as important as the farmers and woodcutters.
At the end of the summer of 650 CE, the emperor prepared to leave his Summer Lodge in the north of the country to return to Ito, the capital of Nihon-Ja, and spend the winter months there. But while he was preparing his journey, the Senshi Arisaka, pater-famillias of the mighty Shimonseki-clan, comitted a coup in Ito, took over the whole southern part of Nihon-Ja and marched his army north. He got supported by most of the Senshi-clans, and his march to the north went quickly.
However, when the Emperor was attacked by some Senshi, he and his followers fled to the north, hoping to find the mighty legendary fortress of Ran-Koshi to fortify there. On his way through the mountains, he travelled through several Kikori-villages, where he convinced the Kikori to join him and go north.