Mahabharat, the world war was being intensely fought. Both sides were continuously making assessments and getting updated with ever changing war situation. There was now growing feeling in the Pandavs camp that as long as Bhishm Pitamah, Dronacharya and Karna were there, the war would not be won. To start with at least one had to be removed. Shri Krishna advised Yudhishtir to return the blessings (Aashirwad) he received from Bhishm Pitamah, now the Commander-in-Chief of Kauravs army, as they were of no use because war could not be won as long as Pitamah was there.
And during the next day battle, following the instructions of Bhishm Pitamah, Arjun, extremely critically and fatally wounded Bhishm Pitamah and ensured that he would no longer have any part in that battle.
You have to decide that in case the commander-in chief of an army intimates the enemy, how he could be removed, would it simply be the case of pledge being broken, or much more than that; maybe amounting to treason, some would say.
In a battle fought for establishing Dharm, there could be compulsion for a person on personal level to choose the side of Adharm. But are personal compulsions/milestone sufficient to warrant destruction of Dharm which is always society centric, and in this case humanity centric? No, NEVER. And this is exactly what Bhishm Pitamah decided. And this is what we too, must follow.