Abstract
'Man is by nature a political animal'. For every realist, Aristotle could be a respectable ancestry more than Machiavelli, who was closer to the classical realist theory. The Realist family is not the oldest, but maybe the simplest one, and the first one to formulate a real answer, and a powerful explanation about international relations. The state of war dominates the international system, even if it is possible to create common institutions. The aim of international relations theories is to understand why the state of war endures. For that, Realists drew upon many different theories to build their own. For instance, 'The Origin of Species' written by Darwin in 1859 underlines the 'struggle for existence', which is one of the key realist concepts . The oldest analysis about war was conducted by Thucydides about the Peloponnesian war (431–404 BC). This writer is often regarded as the founding father of international relations. His writing gives the key point of realist perspective 'The strong do what they can and the weak suffer what they must'. Though a realist perspective had already existed for a long time, the real story of Realism begins after the First World War. The Realist theory was a reaction against the Idealism of the inter-war period. Idealist thinkers, often called 'utopians', believed that states share common interests, and that it could be possible to stop the scourge of war thanks to humankind. The atrocity of WW I made research about peace necessary. The first modern thinkers in International relations were idealists. They came from England and wanted to understand why this war had happened, and how peace could be built by the power of law. The Second World War confirmed the inadequacies of the idealists' point of view, and created a debate between Realism and Idealism (often called the first debate). Realism believes in 'objective factors' , i.e. the world exists without human interpretations, and, consequently, its aim is to explain the world of international politics as it is.
