Why is national defense better off as a natural monopoly? What other industry or service do you think should be a natural monopoly?

What will be an ideal response?

Industries like national defense experience economies of scale. With very high fixed costs, the long-run average total cost falls as output expands. New firms in the market cannot achieve the low costs of the incumbent natural monopolist because upon entry, they are likely to split the market. Such splitting will leave each seller with much higher costs and lower profits. Drinking water, natural gas, and electricity are other examples of industries where it is efficient to have a single supplier in the market.