So, we're safe against cities attacked by enemies! Unless they use UPS and a biological agent, or something. Or get sick and fly here for legitimate business reasons. Or mail the parts for a missile to a bungalow in Burbank and put it together there.
The purpose of a system is what it does; the purpose of the military industrial complex is to funnel government funds into the hands of large private firms. Actual military/defense outcomes are secondary.