Niccolò Machiavelli is considered part of the Realist canon because of the way he introduced a new way of thinking about politics, one which overcame the received wisdom of employing a Christian set of values whilst judging political affairs. In The Prince he distinguishes between virtù and the virtues of Christianity, arguing that the virtù of The Prince should be based upon the security of the State and not on Christian morality. This moment arguably constituted the emergence of Realpolitik and stems from a critique of previous Humanist thought, especially their evasion of the role of power (fortune) in political life.
Machiavelli felt this redefinition of virtù was necessary for the Prince to achieve his noblest ends, that being the successful founding of a lasting, peaceful Republic. He was arguably central to establishing the distinction between positive and normative conceptions of political activity, when he said ‘how men live is so different from how they should live that a ruler who does not do what is generally done, but persists in doing what ought to be done, will undermine this power rather than maintain it’ (Machiavelli in, Chris Brown, ed., International Relations in Political Thought. p 261 italics mine).
While he has been co-opted into the Realist school of International Relations, Machiavelli was generally focused on government within a state, rather than relations between them. Within The Prince, Machiavelli was primarily concerned with the conduct of individual princes, especially in the establishment and maintenance of a state, however in an earlier work, The Discourses on Livy, he was more concerned with offering his advice to the whole body of citizens. In this work, his sympathies lie with “free” forms of government, as opposed to individual rulership. In the Discourses he tells us ‘though but one person suffices for the purpose of organization, what he has organized will not last long if it continues to rest on the shoulders of one man’ (Machiavelli in, Chris Brown, ed., International Relations in Political Thought. p 266). This implies that unlike Realism (classical Realism at least) where the state is considered an opaque and unitary actor, the form of the government of the state is of central importance to Machiavelli’s thought.
Realism’s co-optation of Machiavelli as an amoral player of power politics inverts but continues a tradition of demonising Machiavelli, with his wickednesses turned into virtues. However, a more in-depth reading of Machiavelli reveals a massive amount of context surrounding his advice to The Prince. Therefore, while he should be credited with helping furnish Realism’s canon, he arguably was not a Realist himself.