Theoretical Preconditions of Realism. Philosophical Bases

Realism in International Relations (IR), as a paradigm, is the most developed and widespread of the academic schools in the field. The dispute between the realist and liberal school, being the second most important and widespread schools, can itself serve to elaborate the fundamental principles, concepts, and methods in IR; and from it we can generate recognized rules and define the borders of the science.
The realists offered a range of principles, some of which was accepted by some representatives of the other branches in IR, even if its interpretation differs. In particular the realists introduced two fundamental concepts: anarchy in IR and state-centrism (the State is the only actor making sovereign decisions in international politics).  However realism has based its theories, in turn, on the classical authors of political science's modern epoch, including the founders of modern State theories: N. Machiavelli, T. Hobbes, and J. Bodin. The core of realist theory is based in political philosophy.
The foundation of the realism movement in IR and, more broadly, of a general discussion of the science, is the principle that the modern national State is the main actor. Thus, this entire field in IR is, first of all, the main field (and the only one for the realist) studying international interactions of sovereign States. Theoretically, it comes from three main political and philosophical sources: The Prince by Machiavelli, Leviathan by Hobbes, and Bodin's theory of sovereignty. All of these taken together form the theory of the modern State, which became a basis of the law of Europe (jus publicum Europaeum) and, after the Thirty Years' War, influenced the Peace of Westphalia, from which we get Westphalian sovereignty. So theory became practice. J. Hobson regards Westphalian sovereignty to be the de facto and de jure expression of the fact that the national State is the main actor, the "Big Bang theory in International Relations", stressing that since the start of the Peace of Westphalia epoch, such a view in the State became not only dominant, but universal, the standard, and the only one possible. 
It leads to the conclusion that this field in IR is the field of the relations between national States which are the basic actors. This fact is recognized by representatives of the positivist paradigm of the International Relations, with the fine difference that the realists make, that the State is absolute, and refuse to accept the importance of any other transnational or internal political factors structuring the system of IR. The liberals in IR, on the contrary, fill the list of other factors with those actors: transnational corporations, NGOs, etc. The Marxists in IR believe that  international factors together with class contradictions have a decisive importance, as according to the Marxism, the bourgeois and proletarian classes are international by their definition and this means that their interests are not limited by national borders. 
Now we turn to some details of the theoretical axioms, being the basis of the comprehension of the nature, the functions and the structures of the modern State by the founders political philosophy of the modern age.
The Prince (by N. Machiavelli)
The Italian philosopher Niccolò Machiavelli (1469-1527) is the one of the first modern State theorists. He concentrates his attention on the image of a Prince, but means by this not the traditional monarch whose governing is based on traditions which are ensured by historical, social and political pressures that must be followed to persist, but a new Prince, i.e. such as a politician that may create a State, a political system from scratch, relying on his will and his mind. Machiavelli is interested in this kind of new Prince primarily because he had the goal of basing political creations on rationality, will, and efficiency. 
Machiavelli considers (or creates) a new political object that didn't exist in the Middle Ages and in Antiquity; it is the modern State, being an efficient instrument of social arrangement in the respect of  the leader's (this new Prince) interests;, but its efficiency needs to be demonstrated. The new Prince (contrary to the old one) must not only vest his power in the present society with its deep traditions and orders that he has to follow, but to create a society, the order and, during its creation, strengthen its power and prove its efficiency by his actions. At the same time, Machiavelli regards religion and tradition to be the social constructs of those who previously played the role that the new Prince does now, and had created the political and social traditions being the basis of political and cultural persistence.  This approach of Machiavelli to religion was the reason for the Vatican to ban his work. 
The Machiavellian State has no other purpose except the goal of the new Prince's domination, as all of his institutions and principles must have an absolutely practical goal and also prove their efficiency. The determined character of his governing is such that changes to the political rules are made right at the moment when they are approved by the one or another of the goals. The "end justifies the means" formula, if not in its form, but in its content, could be used in context with Machiavelli, and be used to comprehend the State in the political philosophy of the modern age in general. For instance, Machiavelli, in his other work , Discourses on Livy, defends republican ideas that allow us to understand the political image of the Prince as the collective image of political power, regardless of whether it has an individual (monarchic) or a collective (republican) character. In both situations, it is important that the State is regarded as the totally instrumental mechanism of governing, which  the efficiency of power assumption and its retention by the ruling elite (the new Prince) all emanate from, and the expansion and protection of the national territories is the main objective of the rulers, and is one of the major criteria of efficiency and stability of their governing.  
The feature of Machiavelli's political philosophy which is accepted by the majority of the political philosophers of the modern epoch, is the division of sphere of morality into two components: personal and State morality. As personal morality is traditional and can be rationally explained, State morality on the contrary is based in the efficient achievement of goals and, under some circumstances, this can contradict the norms of personal morality. Thus, Machiavelli states that for power assumption and its retention, in some extraordinary cases, the new Prince can neglect morality: to poison, to swindle, to murder, to violate, to bribe, to lie, etc. can be approved by the power interests themselves or State interests (for example, to defend the territorial integrity). Machiavelli did not encroach upon the existence of personal morality, believing that it has the right to exist and is enough to regulate norms of behavior. For the Prince, however, the dominating morality are other criteria and estimations, rules and processes, that are evaluated by their ability achieve goals efficiently and, respectively, power retention and defense of the State's national interests.
Thus, while the majority of the experts find Machiavelli's ideas to be cynical and amoral (recall that "Old Nick" is used as another name for the "devil" in English political slang), his influence on the political conceptions of the modern age is impressive.  
Leviathan (by T. Hobbes)
The other key theorist of the modern national State was English philosopher Thomas Hobbes (1588-1679). Hobbes, while creating his theory, based in the anthropological pessimism, i.e., the assurance that the natural state of individuals, left to their own devices, is anarchy, violence, egoism, and a will to satisfy their material needs at the expense of others, which are the causes of all the factors in this "war of all against all" . Hobbes has the important formula "a man is a wolf to another man" (homo homini lupus est). The egoistic nature of humans creates obstacles on the path to the creation of a harmonized society, cooperation, and eliminates altruism and solidarity. However, people have intellects. The additional quality that animals do not have (or replace it by the instincts), allows people to realize the destructive power of the natural state and make the conclusion that it is important to create an artificial restriction of the "war of all against all". Thus, according to Hobbes, people find the need for a "social contact", i.e. the fundamental agreement that they all agree together to give up the unlimited, unfettered  achievement of their egoistic needs, in favor of generally accepted norms. But the egoism of the natural state, i.e. of human nature itself, said Hobbes, demands that its restrained by the violence of the institution that is able to prevent it from violent acts. That is why it is established on a social agreement institution that must be coercive, powerful, and intimidating, being able to use violence against those who start to break accepted rules which bring us back to the natural state (anarchy). Thus, Hobbes comes to the concept of the Leviathan, a terrific biblical monster, living in the sea and frightening ordinary people.  Leviathan is the earth lord, created by people in the process of their social interactions to prevent self-destruction, and to restrain anarchy. Leviathan is the modern State.
As opposed to the traditional Middle-Age State and, especially, the Empire, Leviathan has no mission, no spiritual or historical positive goal, no divine fate. Leviathan has no goal to improve human nature, Hobbes believes that it is impossible and unrealistic. Leviathan just prevents the anarchy and self-destruction of all mankind. That is the whole function. At the same time, Leviathan is created at the initiative of the masses, it is handmade, it is the product of human rational conclusions and comprehension of his social (quite frankly, asocial) nature.
Such a meaning of the State becomes one of the main forms of how governing and the polity is interpreted in the modern era, and is more-or-less accepted by the representatives of the Modern political science.
The Leviathan is an instrument of legitimate violence (as is the essence of the State, defined by Max Weber), as it is the product of an agreement and the incarnation of rational human nature overcoming his natural (animal) state. Inside the Leviathan is competence, the society is organized in such a manner that there is no anarchy. The unit performs as the institution of collective rationalism, determining the rules of individual behavior in the realm of interaction, and having the force potential to punish those who go beyond the rules.
Theory of Sovereignty (by J. Bodin)
The third key author of the political philosophy of the modern era is Jean Bodin (1530-1596). Jean Bodin was interested in different spheres: from theology to the natural philosophy and even demonology, but mostly his ideas influenced the sphere of political science: he became the first theorist of sovereignty as a concept. According to Bodin the ruler is sovereign who has no power institution above him, except God. In the 16th century, when Bodin was living and working, the concept was aimed against the Roman Catholic Church, insisting that, in several cases, the policy of the European kings was made to follow the Christian norms, particularly, be in agreement with the Pope on these issues. Bodin showed a completely different (Protestant, in fact) idea of the State that would not have as its goal the "people' happiness" (as it was according to Aristotle and Catholic Scholasticism), but instead to be a rational and artificial arrangement of affairs. The main instrument of such a rational arrangement is the sovereign.