Naze Boku no Sekai wo Daremo Oboeteinai no ka?

なぜ僕の世界を誰も覚えていないのか？

In a world where humans triumphed in a great war, Kai guards sealed crypts containing their enemies. But when “World Rebirth” overwrites history, he’s thrust into an alternate reality where humanity lost the war and he’s been forgotten by everyone he knows. Now, this lost hero must rise to restore the world’s balance!

(Source: Crunchyroll)