The US Dollar reserve system means that the dollar has been the reserve currency of the world since the Bretton Woods agreement in 1944. The American military based abroad(around 900 in 130 countries in 2012) are meant to keep this hegemony going. The Petrodollar( the fact that dollar has be bought in dollar) is also one of the key interests.
This is why military bases are close to oil wells or oil pipes, for instance in Kosovo or Saudi Arabia. Saddam Hussein and Gadhafi were overthrown partly because they threatened that system by wanting to accept other currencies than the dollar to sell their oil.