Eiyuu to Kenja no Tenseikon: Katsute no Koutekishu to Konyaku shite Saikyou Fuufu ni Narimashita

In a world ravaged by war and conflict, Hero Raid Freeden and Sage Eluria Caldwin clashed time and time again over fifty long years as the strongest soldiers of their respective nations. However, their long-standing rivalry came to an inevitable yet bitter end with Eluria's sudden death. A thousand years later, Raid finds he has been reincarnated—and so has Eluria! The world may now be at peace, but the two have an unsettled rivalry between them and the mystery of their reincarnation looms ominously over their heads. There's so much that needs to be done, so they decide to...get engaged and attend the Royal Institute of Magic together?!

(Source: J-Novel Club)