Before a single arrow flies, Bhīṣma, the eldest of the Kurus, roars like a lion and blows his conch. He loved the Pāṇḍavas as much as the Kauravas, and still fought for one side.
Going deeper
1.11 is Duryodhana's instruction to his commanders: take your places in your divisions and guard Bhīṣma at all costs. 1.12 answers it with sound. The grandsire Bhīṣma, the eldest of the Kuru clan, roars like a lion and blows his conch to cheer Duryodhana. 1.13 then fills the air: conches, kettledrums, trumpets and horns all sound at once, and the noise is terrific. That Bhīṣma is first to sound his conch, and that he does it to lift Duryodhana's spirits, is in the verse itself. Later, in 2.4, Arjuna names Bhīṣma and Droṇa as the two he cannot bring himself to aim at. The Gītā shows him first as a loud, steady presence and then as the person Arjuna most hesitates to face. See chapter 1, and what the Gītā says about courage.