The CIA World Factbook puts the US
as the world's 19th biggest spender on defence, although it's chosen to use % of GDP compared via exchange rates and not the usual PPP. That's inevitably going to skew things even before the fact that the figures are for a variety of years rather than one consistent one.
I also find it strange that an organ of the US government specifically devoted to collecting information only has figures from its own nation's expenditure for 2010. Maybe I shouldn't, given their track record.