re: Explain for what.
Couldn't explain to you. You don't believe any military action was necessary anywhere.
The macroeconomic costs in their paper were a large chunk of it. Eliminate those, and other blue-sky assumptions, and you wind up with $1 trillion at most, counting all the veteran issues.
But hey, go with it. I have found that pointing out a number is incorrect has never stopped people from repeating it over and over again.