Seiryaku ~Koukyuu de no Himegoto~

Continent drowned in the flames of war. The king of "Jin" country falls ill and his eldest son takes over. He urgently needs to get a wife to become a proper ruler. Six neighboring countries instantly send their bride candidates. If he gives a clear preference to one of the girls, the rivals will assassinate her the same day. He needs to resolve the situation peacefully while keeping his affairs secret.

[From vndbreview]