Around 30 years ago Congress required all government agencies to be able to pass an "unqualified audit". To date, the Department of Defense is the only agency that has never passed an audit. This with internal auditors. One can only imagine the results of a Big 4 audit. Yes DOD is the largest agency, scattered throughout the world. It would be a monumental undertaking. The general public would have much more faith in the system if DOD could pass a GAAP audit at the same level as most business, organizations, states and other government agencies have to pass.
Even this past year DOD can't account for $1B of military aid to Ukraine. I'm not saying it is graft but it is certainly poor management by DOD at best.
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Regarding Ukraine which it looks like your post was dealing with, I don't know if there is any corruption within the military aid. Last I saw which was a year or so ago, military aid was around half of all aid going from America to Ukraine. USAID and NGO's funded by DOS, CIA, etc is full of problems. I have witnessed graft twice on a small scale regarding US taxpayer money given to a strategically insignificant African country. The $1 billion aid package Joe Biden held up to get Victor Shokin fired for investigating Burisma who was paying $50,000/mth to Hunter Biden plus other monies that haven't been divulged yet is an example. Ukraine was the second most corrupt country in Europe, even behind Russia before the invasion. Is there any reason to think this changed with the advent of billions of extra dollars flowing through the coffers. It is interesting that the Ferrari, Lamborghini and other luxury dealers in Romania and Bulgaria can't keep cars on the showroom floor due to the demand coming out of Ukraine. That is not typical of a country in the throes of war.