The national interest, often referred to by the French expression raison d'État (English: reason of the State), is a country's goals and ambitions whether economic, military, or cultural. The concept is an important one in international relations where pursuit of the national interest is the foundation of the realist school.
History of the concept 
In early human history the national interest was usually viewed as secondary to that of religion or morality. To engage in a war rulers needed to justify the action in these contexts. The first thinker to advocate for the primacy of the national interest is usually considered to be Niccolò Machiavelli.
The practice is first seen as being employed by France under the direction of its Chief Minister Cardinal Richelieu in the Thirty Years' War when it intervened on the Protestant side, despite its own Catholicism, to block the increasing power of the Holy Roman Emperor. At Richelieu's prompting, Jean de Silhon defended the concept of reason of state as "a mean between what conscience permits and affairs require." The notion of the national interest soon came to dominate European politics that became fiercely competitive over the next centuries. It is a form of reason "born of the calculation and the ruse of men" and makes of the state "a knowing machine, a work of reason";the state ceases to be derived from the divine order and is henceforth subject to its own particular necessities (E. Thuau, 1966).
States could now openly embark on wars purely out of self-interest. Mercantilism can be seen as the economic justification of the aggressive pursuit of the national interest.
A foreign policy geared towards pursuing the national interest is the foundation of the realist school of international relations. The realist school reached its greatest heights at the Congress of Vienna with the practice of the balance of powers, which amounted to balancing the national interest of several great and lesser powers.
Metternich was celebrated as the principal artist and theoretician of this balancing but he was simply doing a more or less clean copy of what his predecessor Kaunitz had already done by reversing so many of the traditional Habsburg alliances and building international relations anew on the basis of national interest instead of religion or tradition.
These notions became much criticized after the bloody debacle of the First World War, and some sought to replace the concept of the balance of power with the idea of collective security, whereby all members of the League of Nations would "consider an attack upon one as an attack upon all," thus deterring the use of violence forevermore. The League of Nations did not work, partially because the United States refused to join and partially because, in practice, nations did not always find it "in the national interest" to deter each other from the use of force.
The events of World War II lead to a rebirth of Realist and then Neo-realist thought, as international relations theorists re-emphasized the role of power in global governance. Many IR theorists blamed the weakness of the League of Nations for its idealism (contrasted with Realism) and ineffectiveness at preventing war, even as they blamed mercantilist beggar thy neighbor policies for the creation of fascist states in Germany and Italy. With hegemonic stability theory, the concept of the U.S. national interest was expanded to include the maintenance of open sea lanes and the maintenance and expansion of free trade.
Concept today 
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Today, the concept of "the national interest" is often associated with political Realists who wish to differentiate their policies from "idealistic" policies that seek either to inject morality into foreign policy or promote solutions that rely on multilateral institutions which might weaken the independence of the state.
As considerable disagreement exists in every country over what is or is not in "the national interest," the term is as often invoked to justify isolationist and pacifistic policies as to justify interventionist or warlike policies.
The majority of the jurists consider that the "national interest" is incompatible with the "rule of law". Regarding this, Antonino Troianiello has said that national interest and a state subject to the rule of law are not absolutely incompatible: “While the notion of state reason comes first as a theme of study in political science, it is a very vague concept in law and has never been an object of systematic study. This obvious lack of interest is due to a deliberate epistemological choice - a form of positivism applied to legal science; and as a result legal science affirms its autonomy regarding other social sciences while constituting with exactness its own object - law - in order to describe it. In doing so it implies deterministic causes which have an influence on its descriptive function. This method which puts aside state reason is not without any consequence: the fact that state reason is not taken into account by legal science is to be integrated within a global rejection of a description of law as presented in political science. A fundamental dynamic in modern constitutionalism, "the seizure of the political phenomenon by law" is all the more remarkable when it claims a scientific value, thus a neutrality aiming at preventing all objection. This convergence of legal science and constitutionalism has the tautological character of a rhetorical discourse in which law is simultaneously the subject and the object of the discourse on law. Having as a basis state reason, it allows a reflexion on the legitimacy of power and authority of modern Western societies; this in connexion with the representations which make it and which it makes “state reason and public law”. (Troianiello, p. 690)
See also 
- Sphere of influence
- Realism (international relations)
- Neorealism (international relations)
- Nation state
- "E. Thuau": "Raison d'Etat et Pensee Politique a l'epoque de Richelieu; Paris, Armand Colin, s. d. , "W.F. Church, Richelieu and Reason of State (Princeton: Princeton University Press, 1973), 168; J. Franklin, The Science of Conjecture: Evidence and Probability Before Pascal (Baltimore: Johns Hopkins University Press, 2001), 80-81.
- Byrd, Peter (1996). “The Concise Oxford Dictionary of Politics”. Oxford University Press
- Gvosdev, Nikolas K. (2004), Russia in the national Interest (1st ed.), Transaction Publishers, New Brunswick (U.S.A.) and London (U.K.), pp X,ISBN 0-7658-0564-2, ISBN 978-0-7658-0564-5
- Lavy, George (1996), Germany and Israel: Moral Debt and National Interest (1st ed.), Frank Cass & Co LTD, LONDON, pp XI,ISBN 0-7146-4626-1, ISBN 978-0-7146-4626-8
- Troianiello, Antonino (1999), Raison d’État et droit public, Thèse dactylographiée, Université du Havre, 748 pages