"One could say that, because of living in the same world, the fates of the humans and Ayakashi were tied. If one world had a war, it was mirrored in the other. "
The First and Second World Wars echoed a war that was fought by the Hengen Ayakashi from early 1913 to mid 1945. These Ayakashi wars are now known as the 'Territory Wars', named after the bloody and violent clashes between the Seven Great Clans of the Hengen, with each fighting for more land, which meant more power to them. For 32 years, the Hengen gained, and lost, and gained, and lost again what land they took for themselves, all fighting for larger and more land. The war in the supernatural world was affecting the human world, in the form of the two World Wars, and was upsetting the balance of the world.
Things got so bad that the Kamigami eventually had to step in, initiating the Territory Treaties. The Territory Wars ended with each Hengen Clan keeping what land they had claimed up to the moment the Kamigami became involved as their own. Attacking one Clan without provocation would bring the Kamigami back into the fold to bring down upon the attacking Clan a punishment so severe, it could very well wipe out that Clan.
*note: the Seven Great Clans are the Hengen Ayakashi, which are the Tengu, Kitsune, Mujina and Tanuki, Hebi, Tsuchigumo, Nue, and Ookami. The Bakeneko and Nekomata are Hengen, but they are solitary and spread out over the lands rather than with their own territory. The colours indicated in the map are in no way relevant to the village clans of the Tengu.
The Territory Treaties brought peace back to the Ayakashi world, though ties between the Seven Great Clans remained forever broken, with each Hengen clan distrusting the other. It is unknown what began the Territory Wars in the first place. The Treaty is signed every year by the Kings, the rulers of the each Great Clan, or the Heirs, the princes and princess who are the direct inheritors of the Kings. A Clan having an Heir is only applicable when that Clan chooses to have their Kings ascend to power through bloodline and inheritance, though some Clans choose to elect their Kings.
The map below provides a look into the general Territories each Hengen Clan kept when the Territory Wars ended.
Key: Black - Tengu White - Kitsune Blue - Tanuki Yellow - Mujina Red - Nue Brown - Ookami Green - Hebi Purple - Tsuchigumo