I Was in an Enemy Country, so I Married the Princess in Order to Survive.

He is a down-and-out prince who has not been favored by the emperor since he was a child. Sent to the enemy country as hostages. In order to survive, he had to give up his dignity as a man and be willing to condescend to become a knife in the hands of others. Suddenly one day, he met a girl and found that as long as he married her, he could change his life. So, he set a small goal for himself, such as marrying an eldest princess first.