I think that most of this $8.5 billion might have been spend on secret missions and assignments, just like the recent Kiev intervention. This spending can't be made public, as it will anger a lot many of the allies of the United States (The US regularly back stab its allies, although it tries its best to pull things under the carpet). 
It clearly says Trillion, not billion. $8.5 Billion = $8,500,000,000
$8.5 Trillion = $8,500,000,000,000
Oh.. my mistake. Sorry about it. But it is still confusing. I have heard that the US defense budget amounts to some $600 billion per year. And this $8,500 billion figure represents more than 14 years of total defense spending. No one really noticed this much money missing for such a long time? Difficult to believe all this.
Well, it´s 19 years of "accounting" actually. And Uncle Sam´s Mickey Mouse budgets always have megatons of "off balance sheet" items anyway. It was 2.3 Trillion that they couldn´t account for by 10th Sept. 2001 according to Rumsfeld and you have seen all the war scams since then.