Daiteikoku

Before the year 0 AD United Space Age, humans dotted parts of the vast universe and established independent nations.
Since sailing to other star regions requires more than several hundred years, it was practically impossible to establish contact with other nations.

In the year 0 AD United Space Age, the Eris Empire advanced their research on "strange spaces within star regions", and discovered that you can travel to other star regions in an instant using such spaces.
Such spaces named "warp gates" united the various star regions that have been so far isolated from each other.
Thus began the Age of Discovery.

Human beings who aspired to build nations spanning multiple star regions gradually repainted the star regions network map - the world map, through repeated aggression and conquest, submission and annihilation.

It's 939 AD United Space Age.
The leaders of three great nations are planning to start wars due to various reasons.
The biggest nation in the world, Eris Empire's council passes the world management plan that says Eris should suppress and dominate the chaotic world.
The oldest nation in the world, the Japanese Empire, tired out from the drawn-out war with the Chinese Empire, is being targeted by great nations.
A grand flame of war that will consume the whole world has been lit.