Horizon in the Middle of Nowhere

In the distant future, humanity returns to a devastated Earth after being unable to travel into space due to an unknown phenomenon. Japan is the only habitable region, so the government creates pocket dimensions to accommodate the entire population. The Testament, a holy book guiding a reenactment of human history, becomes the central framework for this endeavor. However, by the year 1648 of the Testament Era, the pocket dimensions' nations invade and conquer Japan, displacing its original inhabitants.