Kyoukai Senjou no Horizon

In the distant future, Earth has been devastated and its inhabitants seek a new home in the heavens. However, constant warfare forces humans to return to Earth, which has turned into an uninhabitable planet except for a certain area called the Divine States.

The Divine States are too small to accommodate all the humans, so they duplicated the area and created the Harmonic Divine States. In order to retrace their steps and rediscover their journey to the heavens, the returned humans began to reproduce history from the year 10,000 BC, using a mysterious history book called Testament.

History reproduction proceeds smoothly until A.D. 1413, when a war broke out in the Divine States. This causes the Harmonic Divine States to crash onto the original world. The humans living in the Harmonic Divine States lose their land and invade the original world. The people of the Divine States surrender and they are divided by the invaders from the Harmonic Divine States. They try to resume the reproduction of history from A.D.1457, but the update of history terminates in A.D.1648. A rumor of apocalypse begins to spread around the world.