Protecting people from hybridity is the eternal duty of kings

In the 57th chapter of Rajdharmanushasan Parva, grandfather Bhishma Maharaj tells Yudhishthir -

 Chaturvarnasya Dharmashcha Rakshitva Mahikshita l
Dharmasankararaksha Cha Ragyam Dharmah Sanatanah ll 15 ll

That is, it is the duty of the king to protect the religion of the four varnas. It is the eternal duty of kings to protect the society from religious hybridity. 
It is clear here that if different people leave following their own religion and start adopting other religions then there will be caste hybridity. Due to this, chaos and anarchy becomes widespread in the society. Inactivity and inaction increase. The smooth flow of work and people's journey is disrupted. Therefore, protection of the people i.e. the society from religious hybridity is the eternal duty of the kings. Making this even more clear, Pitamah has said-

Trayyam Sanvritamantrashcha Raja Bhavitummarhati l
Vrijinam Cha Narendranam Nanyachcharakshanata Param ll 14 ll

That is, the king should rule by having knowledge of the three Vedas and keeping secret policies secret. Protection of the subjects is the supreme duty of the king and there is no greater sin than not protecting the subjects. Here carelessness is said to be the biggest sin in protecting the people. Protection means not only protection of body and property, but also protection of one's own religion and traditions. Because man is not just an enjoyer of body and things. He is also the carrier, nourisher, nurturer and protector of mind and intellect, knowledge and memory, values and resolve, tradition and religion. Therefore, protecting the people means protecting Sanatan Dharma and protecting the people from racial diversity. 
Further in the 16th verse, Pitamah has said that the king should intelligently follow these six qualities – Sandhi, Vigraha, Yan, Asana, Dvaidhibhav and Samashraya. The meaning of the treaty is clear. Vigraha means to weigh one's strength and fight or continue the fight with the enemy. Yaan means attack or campaign. Asana means opportunity to remain calm and wait for the appropriate opportunity. Ambivalence means behaving in a way that creates dilemma. Giving shelter to the enemy's enemy and taking his support is Samashraya. A ruler should follow these six qualities wisely.  In the 17th verse, Pitamah has emphasized on espionage. After this, emphasis has been laid on efforts to protect our treasury well and for continuous growth. It is also said that the king himself should take care of those whose maintenance has to be arranged and take care of everyone -

Abhritanam Bhaved Bharta Bhritanamanvavekshakah l
Nripati Sumukhashcha Syat Smitpurvabhibhashita ll 19 ll

The husband of the immortals should be the observer of the servants. The king should be a good-faced woman with a smile and a smile on her face Further, the king should associate with the wise elders and saints and by observing their behavior carefully, they have been asked to learn from it. Besides, the grandfather also emphasized that the king should always be 'Charudharshan'. That is, their attire should be such and their image should be such that one experiences beauty and happiness upon seeing it. Further Bhishma Pitamah says that protecting the property of the people is the duty of the king. Wealth should never be snatched from the hands of good people. But they should also be given money from the treasury. Financial punishment must be imposed on unscrupulous people who engage in misconduct and the evil ones must be attacked. King by always remaining pure and virtuous should be charitable. Those who are devotees of God, brave, noble, healthy and well-mannered and have self-respect towards themselves, should respect others in the same way and should never insult gentlemen. The king should always make such people his assistants. . The king should appoint as his assistants only those who are knowledgeable about public behavior, scholars, sages, those with unshakable loyalty and those who keep an eye on the activities of the enemies and give them royal treatment. Such saints should be treated the same directly and indirectly. Never doubt your trusted and respected people. Because one day the greedy king who doubts everyone is killed by his own people. The king should always win the hearts of his subjects by remaining pure inside and out. Even if such a king is defeated by the enemy, he quickly rises again with the best helpers.
The king should always stay away from vices and conquer his senses. Do justice to all the varnas and be kind to the dependents. Only the king who takes care of his subjects like his own children is the best. The justice and penal system in the state should be such that thieves and criminals always fear it and gentlemen remain fearless. Because such a king is the best in whose kingdom Sanatan Dharma is followed. One who respects knowledge and knowledgeable people, remains engaged in understanding the subject matter and follows the path of good men. - Further Bhishma says that the one whose spies are such that the enemy the king who can never be recognized is successful. The people wish for such a king above all and first of all. Because the protection done by the king is what sustains the world and there is no Sanatan Dharma other than that.

Tadrajye Rajyakamanam Nanyo Dharmah Sanatanah l
Rite Raksham Tu Vispashtam Raksha Lokasya Dharini ll 42 ll

Then the grandfather says that you should listen with concentration to the two verses that Prachetas Manu has said about Rajdharma -

Shadetan Purusho Jahyad Bhinnam Navamivarnave l
Apravaktaramacharyamanadhiyanmritvijam ll 44 ll
Arakshitaram Rajanam Bharyam Chapriyavadinim l
Gramkamam Cha Gopalam Van Kamam Cha Napitam ll

That is, the king who does not protect, the Acharya who does not give proper sermons, the Ritvija who does not pronounce the Vedic mantras properly, the Katuvadini wife, the cowherd who concentrates only on living within the village (i.e. the one who refrains from grazing the cows) and again and again- These six types of people, who like solitude and solitude, should be abandoned in the same way as people immediately abandon a boat that is filling with water due to a hole in the sea.
Further, Bhishma Pitamah mentions the names of these great political science experts of India: Lord Brihaspati, Lord Vishalaksh, great ascetic Shukracharya, Indra with thousand eyes, Prachetas Manu, Lord Bhardwaj and Munivar Gaurashira. They say that all of them are Brahmin devotees and Brahmist people and are the pioneers of Rajshastra. These religious souls have stated that the supreme duty of the king is to obey the subjects and have mentioned these means of religion for the protection of the subjects - appointing the best spies and ambassadors, paying salaries and allowances to the servants on time, collecting taxes judiciously, ensuring that the wealth of the subjects is never destroyed. Not usurping, efficiency in work, speaking the truth, bravery, collection of good men, constant concern for the welfare of the people, adopting measures to create divisions among the enemy, timely renovation of buildings and temples, taking care of the destitute, appropriate punishment for crimes, protection of gentlemen, Company of the nobles, collection of collectable things, consumption of knowledgeable people, giving rewards etc. to increase the enthusiasm and joy of the army, not experiencing any pain in performing the duty, continuous increase of treasury, defense of the state complete management of affairs, do not depend on others in this matter, if there is any factionalism against you in the state, then divide it, always keep an eye on the enemy, always keep an eye on the friends, keep an eye on the middlemen too, among the servants. Not allowing factionalism to develop, regularly inspecting the capital, assuring justice to all, following Niyati Dharma, being constantly industrious, being alert from the enemy, complete renunciation of the mean and wicked and not trusting anyone completely, all these. They are the means of defense of the state. This is the rendering of these great Rajshastra pioneers.
Further, in the 98th chapter, Pitamah Bhishma propounds the importance of upliftment for kings and cites Brihaspati as evidence in this matter. He says- 'Industry is the root of Rajdharma.' 
Industry here means continuous activity and effort. Devraj Indra obtained nectar through industry and through industry he attained superiority in the world of gods and this world and killed the demons. The one who is brave in industry is the real hero. The eloquent scholars only entertain the industrialist men. They are unable to do any great effort themselves. A king without industry, even if he is intelligent, is defeated by his enemies. Even a weak enemy should never be ignored. One should not treat him as small and show carelessness on his part. Because even a little poison kills and even a little fire burns. If the enemy is equipped with even a part of the army, he can harass even the prosperous king by taking shelter in the fort. Therefore, the king should always remain an industrialist. Always keep confidential things hidden so that people remain confused. Increase your reputation through religious activities and always keep your goal in mind. There should be a wise balance of both softness and cruelty in a king. Further the grandfather says that even if he loses his life while protecting his people, it is still a great duty for the king. The state is an object for everyone's consumption. Therefore its handle is internal straightness and simplicity.
It should always be done with caution. This is Rajdharma.