Santairiku Eiyuuki

Possessing memories of being born, raised and died in a country called Japan... the young boy Hercule was born in the imperial household of a declining empire. This empire that once was a super power that reigned over the world... is now a country nearing it's end. The public finances are on bankruptcy, the military strength declined to the point that barbarians are able to wreck havoc on their territories and the neighboring countries squeezed the Empire for tribute payments every year. And the young Hercule is forced to take over this empire!

"Huh!? I can't run away!?"
"You can't."

Thus, the 12-year-old Hercule ascends to the throne. Using the knowledge from his previous life, Hercule struggles to rebuild the empire... reforming the tax system, revolutionizing the industry, reorganizing the public finances, rebuilding the administration and reforming the military system. This will later be known as the story of the young emperor who restored the Lemurian empire, which would later carve its name in history as the great monarch of the three continents, the "Divine Light Emperor" Hercule the First.

(Source: Shousetsuka ni Narou, translated)