A brief description of the Genjitsushugisha no Oukokukaizouki manga:
Suddenly, Kazuya Souma finds himself in another world – he was called here as a hero. Only his heroic journey… It never started. Having heard from the king about the terrible situation of the country, he offers his plan for its revival, relying on knowledge from the modern world. Soon the king renounces the throne and gives the crown to Souma, and announces his engagement to the princess!