The Mythical Hero's Otherworld Chronicles: Volume 1 by Tatematsuri

Unexpectedly summoned to the world of Aletia from modern-day Japan, Hiro Oguro soon discovers this isn’t his first visit. As the legendary Hero King Mars, he once ruled battlefields, conquered nations, and founded the continent-spanning Grantzian Empire before giving up all memories of his hard-won glory to return home to his old life. Now he’s back, a thousand years after he left, and his legacy is in peril. The Grantzian Empire creaks under a warmonger’s rule while ambitious royals squabble for power and foreign nations sharpen their swords. After a chance encounter with Princess Elizabeth von Grantz draws him into a cutthroat struggle for imperial succession, Hiro must don the mantle of the War God once more. Can he live up to his own legend now that Aletia needs him again?