As the concentration of power in the hand of few is not proved in a free market (quote me one company that ever got enough capital for mass scale coercion), it has been already been proved in democratic (and non democratic) government.
Large U.S. oil companies have significant influence on the government and many wars were ignited by them in last 20 years.
None of the wars were fought because of oil companies in the last 20 years. Some wars were fought over oil, but not the companies themselves.
Wars no, but some firefights, mercenaries shooting natives, yes.