Kyoukaisenjou no Horizon

Humans leave Earth for the upper world "Tenjo" after the planet becomes uninhabitable. A war causes Tenjo to collapse, forcing humans to return to Earth, now only livable in the Shinshu region. Shinshu is too small, so a duplicated world called "Juso Sekai" is created. To restore Tenjo, humans begin reenacting history from 10,000 BC. When the simulation reaches AD 1413, a war in Shinshu causes Juso Sekai to fall into the original world. Its inhabitants, now landless, invade Shinshu. The people of Shinshu surrender and are divided among the invaders. The history simulation resumes from AD 1457 but ends in AD 1648, after which rumors of an impending apocalypse begin to circulate.