There's a school of thought that holds that one person's advantage must mean another's detriment - which means the net change for the system is zero (ie, zero sum).
Zero sum theories have been proven wrong time and time again. Where would all the development that has occured in the Western world have come from if it all had to be 'stolen' from elsewhere? Two countries can have a mutual relationship where they both gain from each other (OPEC/ASEAN etc)
There's a school of thought that holds that one person's advantage must mean another's detriment - which means the net change for the system is zero (ie, zero sum).