O King, no one can know the plan of the Lord [Śrī Krsna]. Even though great philosophers inquire exhaustively, they are bewildered. SB 1.9.13
Bhīṣmadeva was certainly satisfied by seeing the triumph of virtue, and he was glad to see King Yudhiṣṭhira on the throne, although he himself fought against him. Even a great fighter like Bhīṣma could not win the Battle of Kurukṣetra because the Lord wanted to show that vice cannot conquer virtue, regardless of who tries to execute it. Bhīṣmadeva was a great devotee of the Lord, but he chose to fight against the Pāṇḍavas by the will of the Lord because the Lord wanted to show that a fighter like Bhīṣma cannot win on the wrong side.
The Hare Krishna Movement 