During peace-time, I think there's a lot to be said about the ROI for the military, if the military is at some reasonable scale (which I think it is not).
With regards to the many Middle East (ME) wars, I think a lot of that is about maintaining and enforcing the petro-dollar. If the ME decides to start selling their oil in Euros or gold or some other currency, it will mean the end of the USA as we know it. The dollar will collapse and China or Russia will take over as the world hegemon. Many people really have a distaste for US hegemony, but unfortunately the real options on the table are not US vs nobody. It's US vs probably China. So the ROI is much more complicated in that regard.
IMO there is no reason we should be killing people outside our borders or even have military outside our borders when there are much larger domestic problems that are unaddressed. I walk by homeless people shooting up right next to million+ dollar houses (yay Seattle + AMZN/MSFT) every day and fixing this shoes not seem to be the priority as the defense contractors and American people themselves are from this porkbarrel.
Yes, war is a racket.
What is your hypothesis on what happens across the globe in the absence of US hegemony and crazy wars. Do you believe a loss of the petro-dollar would lead to US collapse? Do you believe Chinese hegemony would happen, and if so, would it be better or worse than US hegemony?