The chapter ends the way a good commander ends a briefing: with a clear order. Know the Self that stands beyond the intellect. Use that knowledge to steady your mind. And then — conquer the enemy that has been tormenting you from within.
Krishna calls this enemy 'durasadam' — difficult to overcome, but not impossible. The Atman is more powerful than desire. It is more subtle, more enduring, more real. Once you are rooted in that awareness, desire loses its grip. The fire of craving cannot burn what it cannot reach.
This is where Chapter Three arrives after its full journey: from Arjuna's confusion about action and knowledge, through the yajna-cycle, through the examples of Janaka and Krishna himself, through the philosophy of the gunas and the Self — all converging on this final call. Steady yourself. Know who you truly are. And fight.