Grand Duke of the New Empire

The protagonist transmigrates into the body of the son of a marquis in another world, inheriting the marquisate's title, developing the marquisate's territory, leading surrounding nobles, fighting against his own kingdom and neighboring countries, and ultimately becoming a grand duke. The protagonist has no system, no special abilities, and no harem; he conquers the world solely through his 21st-century knowledge and millennia-old Chinese military wisdom.