I generally agree with you that there is no direct evidence of "tribute collection." However, if you look at the most recent example of Iraq, we could make the case that the U.S. has placed permanently placed its military in a country with a lot of oil reserves and could assure itself of cheap plentiful supply from not only Iraq but from the region. Putting my conspiracy theory hat on I would say that the Iraq war was not about WMDs but using it as a excuse to go after the oil reserves.