Q. Bhishma Pitamah, knew that Krishna is the Supereme Lord. Then why he could not stop the Kauravas to fight the war, the result of which he knew in advance?
A. If you read Mahabharata carefully you will find at so many places, Bhishma is giving advice to Dhritrashtra, who was the king and if he wanted, he could have stopped the war. But Dhritrashtra never listened to him because of his blind attachment to Duryodhana.
He had to fight on behalf of Dhritrashtra because he had taken the vow that he will fight for whoever is ruling Hastinapur.
And Lord put Bhishma in this situation to show the world that whoever takes the side of irreligion, no matter who he is will be destroyed.