Realism is a theory of international relations, which is to say that it is a way of understanding relations between countries. I do not intend in this write-up to explore the academic theory of realism, which is often dry and esoteric. The basic idea of realism is incredibly simple to grasp and was expressed by one of the earliest historians; this has endowed it with what its proponents see as a timeless nature. Realism, they say, is rooted in human nature, a nature that is timeless. Their theory has an immense plausibility, also endowed by its simplicity, but also - like all generalizations of human conduct - imperfections.
The basic idea of realism is that in relations between countries, what matters most of all is power. Might does not make right, but it exposes the irrelevance of what is right. Thucydides, the historian who is held to be the father of the theory, wrote that: "The strong do what they can and the weak suffer what they must." While this is not a situation that sounds pleasing to us, it is indisputable that it is an accurate description of a great deal, if not all, of what goes on in relations between states. Realism is an essentially pessimistic worldview because it denies the possibility of transcending this state of affairs and recommends instead that we accomodate ourselves to it.
Realists hold that the driving force of the international system is power and the ability to use it, and draw a great number of inferences from this basic fact. Because international relations takes place in an essentially lawless environment - one in which war is accepted as the acceptable final recourse in a dispute - countries always have to worry about the capabilities and intentions of others and react accordingly. They enter arms races and form alliances because of this basic insecurity, and these actions themselves can contribute towards war because they, in turn, make others insecure. This is called the "security dilemma", and realism sees little hope of ever transcending this either. War is hence inherent in the structure of international relations and an inevitable fact of life.
To a realist, the job of the statesman is to be fully aware of the facts of the environment his country lives in - essentially the military capabilities and intentions of neighbours - and steer the ship of state appropriately so as to advance its interests. Sentiment should not enter into it; how one would like the world to be, as opposed to how it actually is, should be irrelevant. And so opponents of realism have stated that the theory's pessimism becomes a self-fulfilling prophecy, as it denies the human ability to structure the world in ways that improve it. These opponents, who are grouped under the label "liberals" (which has a different meaning to the one it carries in domestic politics) argue that states can decide to transcend constant competition and instead forge agreements that remove the constant threat of war. The success of the European Union has strengthened their argument.
Yet realism's continuing plausibility and explanatory power, if not for all inter-state relations but a great proportion of them, rests on the problem of mistrust between states. A state can only make an agreement with another state if it trusts it, and states always remain concerned that their partners could change their minds. Russia's sudden change of heart about the sovereignty of its neighbours shows why a country always has to base its policy ultimately not just on the intentions of other countries at a given point in time, but ultimately their capabilities to use power; intentions can change at the drop of a government, and military capabilities forever remain a potential threat for as long as they exist.
The only reliable way to transcend the logic of realism has hence proved to be to build trust through reducing capabilities. The European Union has allowed European countries to bring their military force levels to historical lows, whereas the common market means that no country can any longer afford to wage war on the other because their economies are so interwoven. Similarly, after the Cold War, Russia and the West agreed a treaty that set limits on the level of military forces in Europe and agreed to mutual inspections. By monitoring each other's level of military capabilities, the two sides learned not to let dark fantasies dictate their policy, as they did just before the Cuban Missile Crisis when there was widespread fear of a "missile gap" between the U.S. and the USSR, which turned out to be illusory.
Ultimately these agreements can be undone, but they prove that those who claim that there can never be an escape from realism are, at the very least, simplifying matters. In contexts where it is possible to build trust and agreements can be concluded, war can be staved off by the human capacity for ingenuity. But the realist's complete banishment from the classroom of international relations remains a fantasy for so long as states have contradictory interests and live in a finite world, and history shows that the theory's terrifying logic always ultimately prevails over periods of peace. Many generations of Europeans have shared the conceit that they were different and had forever banished war, and they were wrong. We do not know if we are different. What we do know is that the costs of being wrong are incalculably higher than they ever were before.