Realism and the Sovereign State

If we combine together the principal positions of  three classical authors of political science (Machiavelli, Hobbes and Bodin), we will have the fundamental structure of the field of IR. Practically all the representatives of the classical (positivist) theories agreed on it, in general. But the realist characteristic of IR is that they regard the structure as constant, invariable and standard. According to them, all three - the Prince, Leviathan, and the sovereignty constantly and invariably point to a learning which is seen all interactions in IR. It means that for realists:
– the only subject of IR is the national State created on the basis of a social contract, determined by the need to self-defense from egoistic individuals, and to make order in the natural predatory state of the anarchy (Leviathan);
– the national State is governed by the political elite (the "Prince") working on the basis of efficiency, will, and for power retention, based on a morality that differs from the generally accepted one, if one at all;
– the State is absolutely sovereign, i.e. has no institution over it which may be able to limit its freedom of action or make it take into consideration anything, except the national interests.
All three form the basis of the realist comprehension of IR. Dealing with an actor like the modern national State, describing its own changes theoretically by Machiavelli, Hobbes and Bodin, it is possible to create the theoretical model of IR. 
So we come to a very important matter: all the main political philosophies of the modern age are based on the consideration of one State and create their theories from the center to the periphery, where the State borders are situated. The logic of the realists is structured too: the State is the main actor of IR, as it conceives itself, starts from itself, and not from anything else. The priority of domestic policy over foreign is at the root of the sovereignty phenomenon (as well as political power and legitimate violence). The national State is considered as an absolute unit, independent, and founding its activity on fully self-governing logic.
However, the specific character of IR is to consider not just one actor in the field, but many of them in their interactions. It demands new conceptualizations.
International Anarchy
The realist school of International Relations makes, on the basis of the political science of the modern era, further conclusions. The sphere of IR as the relationship between sovereign States, is the field of anarchy (the chaos), where each actor (the State) performs only in respect to their egoistic interests. If, according to Hobbes, the anarchy inside the Leviathan is removed by the Leviathan itself, outside on the contrary, the anarchy exists. That is why war between States is a natural manifestation of IR, nature as the battle between Leviathans. There is no higher institution over the sovereign State, thus it is guided only by its interests, wills and means in relations with the other States. The interests, wills, and means are calculated and implemented by the political elite, i.e. the "Prince", being able to prove his governing efficiency, to retain power and achieve (by any means) his objectives. Thus, the foreign politics of the State become a part of the responsibilities of the institution in the personification of Leviathan, i.e. the carrier of the sovereignty, or the "Prince" (individual or collective). There are no other factors required to identify nature of this IR field: it is the field of independent powers (the States), the wills and interests of which are limited by the contractions and potentials of the other players. On the contrary to the domestic political system, there is no Leviathan (whose decisions are compulsory to observe), no sovereignty (that the other must obey), no "Prince" who makes use of national elites and to dictates to them their goals and methods. There is pure anarchy, chaos. And it is the other side of the orderly, rational, and strict arrangement of society at the domestic political level.
Peace of Westphalia: an Historical Moment in the creation of a New International Order in Europe and in the World
The theoretical position, according to the majority of the classical philosopher of IR, became real after the Peace of Westphalia, when the first diplomatic congress was hosted in Europe and two peace agreements, the Treaty of Osnabrück and the Treaty of Münster (May 14 and October 24, 1648), ended the Thirty Years' War that divided Europe on religious grounds (the Catholics against the Protestants). It is also the year when the Eighty Years' War between Spain and its former- province, which had a religious background too, was finished. The agreements influenced the Holy Roman Empire, Spain, France, Sweden, the Netherlands and their allies represented by the Holy Roman Empire dukes. The Peace of Westphalia created new principles of the European international order based now on the recognition of the sovereignty of all States despite their religious differences. Thus it was a system that completely corresponded to the theoretical structure of the classical political philosophers of the modern era: their ideas were confirmed by the representatives of the most powerful European States, and became the basis of the real arrangement of the political space in Europe. 
At the same time, as the modern representatives of historical sociology IR note, firstly, the sovereignty principle fully corresponded only to the European States, it didn’t correspond to the colonies, and non-European countries (for example, Asian) States were regarded as having “default” or “divided” sovereignty. However, while IR as a science is completely Eurocentric, this fact did not change the general structure at all; it performed normally with the basic actors with national States which are thought of as sovereign by default. In the case of European States, standards and facts were mostly harmonized, and other cases were regarded as “backward” and “anomalous” things that must be improved (through thought the modernization).
The Westphalian system, in its theoretical position, recognizes the sovereignty of all national States strengthening the European space and, little by little, becomes the basic model of the whole world's political structure. The principle of the national States' sovereignty is the main principle of the modern world political structure and, as a result, the Westphalian system is still around today.