Regarding the economy - well...
Here's my take on it: War. War is bad.

Why is War bad for the economy? Is it because people go overseas and get paid money by the government and don't spend their money at Starbucks? Not really.

Here's why it's bad: Energy waste.

1 soldier dead: Probably thousands of gigajoules put into this human in forms of food, education, transportation, etc. When this solder is dead, all the invested energy goes to waste.
1 missile fired: Probably similar to a human - not only does one invest a lot of direct energy to produce a missile with fuel and explosives and components - the energy involved in developing, designing and quality assuring the missile dwarfs its actual production investment.
A few hundred thousand soldiers, driving around in vehicles all day long: LOTS of energy.


You see.. when you wage war, you are actually using your saved up energy to blow shit up, and to kill people, who hold LOTS of abstract energy that is completely wasted when they die.
Your energy is spent on basically nothing. It's just dispersed as heat and nothing in the atmosphere.


Edit: The only good thing about war that I can see, are scientific spin-offs.