Shinwa Densetsu no Eiyuu no Isekaitan

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?

(Source: J-Novel Club)