The Bhagavad-Gita was spoken on the battlefield and so one of its teachings is to know when self-defense is necessary. It was the will of Lord Krishna that all the soldiers on both sides, except for the Pandavas, would not go back home. It made no difference whether Arjuna took part in the war or not. Arjuna was merely an instrument that Lord Krishna used to destroy the soldiers on both sides. Even before the war started, Lord Krishna showed Arjuna, the soldiers being destroyed.
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