If we can hire contractors to rebuild Iraq, why can't we do that in the United States?
Also, the largest employer in the World is the United States Department of Defense with about 3.2 million (plus contractors!). What would be wrong with going into the murder capitol of the United States, and defending American lives? And if we need the "foreign invader" angle, then let's go find all the heroin and cocaine dealers in Oakland, CA and Newark, NJ and put them out of business, because those drugs are provided by "foreign invaders". While we are at it, we can search the mountains of California for marijuana growers employed by the Mexican cartels. We have enough troops to make American MUCH safer here at home for all of us.
And if American were ever invaded, wouldn't a better power grid and broadband internet help us coordinate our defense? Let's put troops to work on things that would both defend our country in case of attack, and at the same time strengthen the infrastructure.