Realism is the predominant
school of thought in
international relations theory, theoretically formalising the realpolitik statesmanship of early modern Europe. Although a highly diverse body of thought, it can be thought of as unified by the belief that world politics is, in the final analysis, always and necessarily a field of conflict among actors pursuing power. Crudely, realists are of three kinds in what they take the source of ineliminable conflict to be.
Classical realists believe that it follows from human nature,
neorealists focus upon the structure of the anarchic state system, and
neoclassical realists believe that it is a result of a combination of the two and certain domestic variables. Realists also disagree about what kind of action states ought to take to navigate world politics, dividing between (although most realists fall outside the two groups)
defensive realism and
offensive realism. Realists have also claimed that a realist tradition of thought is evident within the history of political thought all the way back to antiquity, including
Thucydides,
Thomas Hobbes and
Niccolò Machiavelli.