The Tech Powerhouse in the Mountain Village

An alien's accidental journey through time sends the protagonist back to the past. As compensation, the alien gives him a cheat.

He keeps a low profile as he quietly develops his holdings: digging, farming, mining, smelting steel, making guns, casting cannons, and building planes, missiles, submarines, and torpedoes, then using twenty-first-century weapons to fight World War II.

When he kills, he kills to his heart's content; when he wipes out Japan, he wipes it out completely. But first, he keeps his tail between his legs and stays under the radar.