They Called Me the Useless Prince

Leonard is engaged, and the woman who says she is his fiancée is not happy about it. She accuses him of an affair with her cousin and asks to break the engagement, and he agrees. Leonard is a modern man who has landed in the past in the body of a highly placed heir, and people take him for someone called the Persimmon Prince and misread what he is doing there. When they meet again he is taunted and mocked as a clown. He picks a horse that cannot run, which brings on more mockery, and they tell him the animal could not keep up with a pig and that he will lose his manhood in the spring race, and advise him not to enter. He wins it using modern technology, and after that people stop treating him as a useless heir. He goes on to help the country through several crises, which changes the empress's view of him. Then the court splits over the Fusang navy. Some argue for making peace with Fusang for the country's sake, others push for Leonard's execution as a traitor. The Fusang fleet keeps coming closer while they argue.