This is a major pivot. Having spent 20 verses explaining the eternal soul, Krishna grounds the philosophy in reality. He introduces 'Sva-Dharma' (personal duty based on one's nature). Arjuna is a Kshatriya (a warrior/administrator). Krishna tells him that looking strictly at his Sva-Dharma, he has no right to tremble or hesitate (na vikampitum arhasi). For a warrior, there is no greater good (śreyas) than a righteous war (dharmyād yuddhāc).