This Only Cani Say — Peter G. Bailey

When an asteroid devastates the Earth, a dedicated US naval officer balances his ethical duty against the opportunistic greed of ambitious survivors. With world order destroyed by an asteroid, tough, youthful Marcus Kelloway finds himself opposed to his acquisitive president's policy of taking over countries unable to defend themselves. Asked to assist in the take-over of China, he opposes the operation and begins a nuclear exchange that finally destroys him and the remnants of the world's civilisation.