In the modern world, large scale human military operations are quite expensive (emphases mine) ...
... For the cost of a single additional soldier stationed in Afghanistan for one year, we could build roughly 20 schools there ...The 1 soldier/20 school ratio is a reflection of both the cost of the soldier and the low cost of Afghan construction. It does not include the cost of operating the schools but the point is well made. Our army of one is very expensive.
This is curious because other military actions are getting cheaper. The cost of destruction (aka "cost of havoc") has fallen dramatically over the past few centuries. Even very poor people can afford very effective weaponry, command and communications infrastructure, spy satellites, and even weapons of mass destruction (an interesting variant is the low cost of climate engineering blackmail).
The low cost of certain kinds of military action may dramatically increase the cost of occupation-class operations, particularly those where soldiers can make choices. Separately, soldiering is increasingly a high skills occupation -- and one that's very hard to outsource to a low wage nation. Non-outsourcable high skills occupations are increasingly costly.
Perhaps one of the lessons of Iraq and Afghanistan will be that no future nation will be able to afford the cost of occupation.