Kyoukai Senjou no Horizon

In a far-future world, humanity returns to a devastated Earth after the upper world of Tenjo and the law of causation collapse in war. Only the Shinshu region is habitable, so people create a duplicate realm, Juso Sekai, to make room for everyone. To rebuild Tenjo and causality, humanity begins recreating history from 10,000 B.C. When the process reaches 1413, war erupts in Shinshu and Juso Sekai falls into the original world. Its people, displaced from their land, invade Shinshu, whose residents surrender and see their territory divided. The attempt to resume history from 1457 ends when the record stops updating in 1648, and rumors of an apocalypse spread.