From the introduction:
There is a war. A war lasting centuries, an attempt to control the entirety of the land of Ransei. Legend has it that the man who conquers Ransei will meet Arceus, and control its power. A man named Oda Nobunaga is intent on conquering Ransei, for his own nefarious purposes. The disparate Warlords of Ransei refuse to put aside their petty bickering to fight him, the common people of the land have no ability to Link with Pokémon and cannot stand up themselves. Fortunately bloodshed is averted in the land through the institution of the Battle system, in which if a Warlord loses the battle they lose control of that kingdom. Up to six Warriors and their Pokémon may challenge the reigning Warlord for control, and each Warrior may only challenge a Warlord once per month. This peace is tenuous, any breaking of the System may lead back to a conventional war of armies.