10 years after the Great Tokyo War, Japan had split up into 10 independent nations. Each nations are ruled by a prophet called "Best" and an army called "Rest" as they start to compete to take over other countries. Some Best's are stronger than others and there are bitter turf wars all over the place.
Great Tokyo War preceded the incident. Japan as we all know it was once a unified nation. An unknown clash between some parties had perhaps caused the major problem to rise.
Japan was formed by Emperor Jimmu on 11 February 660 BC. A major force in World War 2, it conquers most of Asia and surrenders in 1945. Japan progress swiftly as a economic powerhouse until one day, there was a Great Tokyo War, when recovering from the war all the wealthy people and ruling class (assumption is government, tribal, heads of powerful households etc) disappeared essentially leaving people to themselves. 10 new states were formed and they divided their territories using the guidelines of the old prefectures. However (and this is unexplained) they gained independence. Each region started becoming nationalistic. As a result of this conflicts with other regions began to arise. The Twin Tower Declaration was created to solve conflicts. Essentially conflicts could be settled through negotiation but if they can't a representative of each vigilante group from the respective states would battle each other (a best) and the winner wins the conflict. It's likely there are states that do not follow the twin tower declaration given the wording. The fight we are seeing at the start is because apparently people who were once tokyo were forced to leave due to the expansion of a company named "Always Comina" and want to reclaim the land through force.