in what way does the narrator use a machine as a metaphor for war

The narrator uses this metaphor because the men fight automatically like machines.... there is no thought or emotion..... there's no time for thought or emotion. They simply fight without thinking, as the only thing that matters at that time and moment is survival.

The battle was like the grinding of an immense and terrible machine to him. Its complexities and powers, its grim processes, fascinated him. He must go close and see it produce corpses.