Realists thought advocates that a state should not be dependent on another state by any means, thus it becomes a Unitary rational unit, examples could be derived from the time of Industrial revolution i.e, Britain, France, Spain required raw material to run their industries effectively, however, since they did not want to depend on each other because they were bitter enemies and competitors in race to power, they headed in different directions to seek the raw material. Realism basically is a zero-sum game. If it is good for me it is bad for you.

Liberalism on the other hand promotes mutual cooperation in order to achieve goals, so inter-dependence does occur, and hence the nations cannot be considered as unitary.