[Noumenon72]

The "global pool to be allocated to anything, anywhere" is called the economy. When concrete is not needed for walling people in, Iraqi concrete factories don't work extra shifts. The people who would have had to work there become employed at other jobs.

In the short term, a soldier who doesn't have to neutralize Sadr City probably does get assigned to some other war mission. In the medium term, his unit is not called up again and he returns to auto mechanic or whatever other productive thing he can do. He puts his effort into raising kids instead of raising barriers.

War does have an opportunity cost.