Because Arjuna refuses to act - he won't fight his friends, relatives and gurus, the epic story of the Mahabharata slides to a standstill. Without action, no narrative is possible; indeed no life. Krishna in the Gita provides the kick-start by stressing the imperative need to act. Very simply, the Gita is a compressed analysis of the three kinds of action possible in human affairs : ritual action (for the physically inclined), reasoned action ( for the intellectual) and spirtitual action (for those inspired by relegious devotion).
The key shloka of Canto 1 is the last (47). Arjuna, stricken by paralyzing sorrow, swirling in indecisions's quicksand, throws away his bow and quiver, and slumps down on this war chariot.
He gives three reasons for his suddenly discovered 'pacifism'. One : sva-jana (one's own people) are to be respected and loved, not 'wasted'. Two : others, blinded by greed, may go in for kula-kshaya (family ruin), but mutually assured destruction is not the civilised way of responding to aggression; certainly not his way. Three : killing is the ultimate crime; better be killed weaponless (ashastram) than kill, whatever the context of the contention.
No wonder Mahatma Gandhi treated the Gita as his 'mother', for here is the core of the philosophy of unarmed resistance, even at the risk of losing one's life. Satyagraha, after all, is 'soul-force'. Those who take up the sword, warned Jesus, shall perish by it; but does it follow that those who die swordless in battle shall find life everlasting ?