The Iraq war was plagued by accounting problems: pallet-loads of dollar bills being shipped in to pay for rebuilding, then vanishing. I don't think this was ever dealt with.

The large military spending is supported by two factors. The first is the element of American morality that believes in righteous violence: it's not merely an unfortunate necessity but a positive moral act. The same attitude supports both individual people keeping revolvers under the pillow and a globe-spanning military.

The second it the lack of inter-state redistribution. American politics since Roosevelt is very against Keynsianism. Suggest paying people to dig holes and fill them in again and there will be outrage. Paying billions for weapons systems that are never used or equipment shipped to Iraq/Afghanistan and then destroyed new, unused? Fine.