Bhagavad Gita 1.12
तस्य सञ्जनयन्हर्षं कुरुवृद्धः पितामहः । सिंहनादं विनद्योच्चैः शङ्खं दध्मौ प्रतापवान् ॥
tasya sañjanayan harṣaṃ kuru-vṛddhaḥ pitāmahaḥ | siṃha-nādaṃ vinadyoccaiḥ śaṅkhaṃ dadhmau pratāpavān ||
Translation
Then, to cheer him, the aged grandfather of the Kurus, the glorious one, roaring like a lion, blew his conch loudly.
Reflection
Where are you doing the thing you spent months arguing against?
Read this verse in its chapter: Chapter One
Bhishma did not want this war. He spent his life trying to prevent it. Now he blows his conch to cheer the grandson he could not raise differently. The roar is for Duryodhana's benefit, not Bhishma's own. People in long obligations do this all the time: they perform the role expected of them on the day it must be performed, regardless of what they argued for in the months leading up. Bhishma sounds the conch because that is what the grandfather of the Kurus does at the start of a battle. The wanting and the doing are separate questions.