Realism is the international relations (IR) theory that is probably the most widely recognized by the general public. It is associated with such familiar terms and concepts as geopolitics, balance of power, realpolitik, and raison d'etat. Realism is known for its emphasis on, and is often conflated with, rationality. It presents an unsentimental and pessimistic view of international relations as being in a perpetual state of conflict and war where power, security, and material wealth and capabilities motivate states far more than ideas or values.

• States are the most important actors in international relations.

• States exist in a condition of anarchy. This condition is comparable to the state of nature in which man existed prior to civilization and government. With no world government or higher authority to enforce rules and claims, states must look out for themselves and assume the worst.

• The international state system is characterized by great uncertainty. States can never be certain about the capabilities or intentions of other states - even those of ostensible allies.

• States seek material power and capabilities in order to survive in an uncertain and anarchic world. Militaries, industry, natural resources, strategically-located and strategically-endowed land or territory, populations, and economic wealth are all manifestations of such material power and capabilities.

• States experience a security dilemma. A security dilemma is a dynamic in which states spiral toward conflict, often unintentionally. Anarchy and uncertainty create the incentive for states to acquire military capabilities and to increase their power. One may do so for purely defensive purposes, but because states cannot know whether another's intentions are defensive or offensive, a state's acquisition of military and other capabilities will elicit concern and reaction in other countries. This dynamic can lead to arms races and to war. The ancient Greek author and historian Thucydides describes a security dilemma in his classic work History of the Peloponnesian War. Thucydides has been called the first Realist; he explained that "what made war inevitable was the growth of Athenian power and the fear which this caused in Sparta."

Four Types of Realism

Among the several points of debate within Realist scholarship, two debates are especially worth introducing and briefly explaining. From these two debates emerge four "types" of Realism.

Classical versus Structural Realism

The first debate is between Classical Realism and Structural Realism (more commonly known as Neorealism). Classical Realism focuses on human nature to explain the high frequency of conflict and war in international relations. Classical Realism holds that statesmen possess reason and generally channel it toward the material interests of the state. However, the importance of individuals opens the door to influence by less laudable aspects of human nature, such as lust for power and glory. Conflict and war are a result of man's - and by extension the state's - search for wealth, power, and glory. States of course seek to survive, but Classical Realism places primary emphasis on the pursuit of power. The most eminent Classical Realist of the 20th century, Hans Morgenthau, argued that states define their interests in terms of power.

Emphasis on the role of human nature and individual leaders is one of the main issues for which Classical Realism has come under criticism as a coherent and useful lens through which to understand international affairs. How can one say that the impersonal state is the most important actor in world affairs but then turn to human nature to explain events? An alternative Realist approach focuses not on human nature, but on the structure, or environment, in which states find themselves.

Kenneth Waltz revolutionized Realism in 1979. Waltz introduced Structural Realism in his influential work Theory of International Politics. Structural Realism, more commonly known as Neorealism, does not look to human nature to explain why conflict is so common. Instead, Neorealism focuses on the structure of the international state system. States do not engage in conflict because of human nature, but because their environment creates incentives for them to do so. As noted earlier, states exist in anarchy characterized by a security dilemma and great uncertainty. This structural environment encourages and discourages certain kinds of behavior. Neorealism is very similar to Darwin's concept of natural selection. The ultimate objective is to survive. Security - not power or glory - is the main concern of states. Power is a means to this end rather than an end in itself. Why is Neorealism important? By placing states in a broader context and by explaining the incentives and disincentives that this context/structure provides, it is easier to make general predictions about how states behave. Neorealism removes the human element from the equation and gives Realism a purer emphasis on the impersonal "State" as the most important actor in world affairs. Statesmen are interchangeable in Neorealism. To Neorealism, it does not matter whether Bush or Obama - or Ahmadinejad or Mousavi - is in office.

Overall, Neorealism is a simpler theory than Classical Realism. But in its simplicity also lies its limits. Neorealism is useful for making general claims about how states should behave, but it does not offer guarantees or specific explanations of how states will act. First, it is perfectly possible that, though the structural incentives tell it to do one thing, a state will do something else. Though states are advised to act rationally and obey the rules of the jungle, Neorealism offers no way to predict if and when they might not do so. Secondly, while Neorealism provides guidelines for how states act, it cannot explain or predict whether and why a state will choose "Policy X" or "Policy Y" within those guidelines. Neoclassical Realists say that Neorealism has moved too far away from the study of statesmen and internal state dynamics such as foreign policy formation. In short, Neorealism simplifies matters and offers context, but leaves a great amount unexplained.

Offensive versus Defensive Realism

Another debate pits Offensive Realism against Defensive Realism. This distinction is more straight-forward than the debate between Classical and Structural Realism. Offensive Realism holds that states are inclined to go on the offensive, wage unprovoked war, and accumulate as much power as possible in order to ensure their security. Defensive Realism holds that states are more inclined to sit back, accumulate only as much power as is needed to ensure security, and work to preserve an existing, stable balance of power.

John Mearsheimer is the leading voice of Offensive Realism. Mearsheimer claims that all states seek to be a regional as well as a global hegemon. Hegemony (pronounced huh-jem-uh-nee) refers to domination and authority short of empire. It is preponderant influence that can be manifested politically, militarily, economically, culturally, or ideologically. The United States today is widely considered the global hegemon. In order to achieve hegemony, according to Mearsheimer, states act aggressively to maximize their power relative to others. States especially seek to prevent the rise of other regional hegemons which may one day grow into challengers for global hegemony. Rather than preserve a balance of power including several countries, states act to skew the balance of power in their own favor, even to the extent that there is no actual "balance" left.

Contrary to Offensive Realism, Defensive Realism holds that states act much more conservatively so as not to create enemies and exacerbate already-troubling security dilemmas. Rather than achieve hegemony, states try not to upset existing, stable balances of power, even if more relative power could be gained in the short-term by doing so.

Is Cooperation Possible?

It would seem that Realism precludes the possibility of cooperation between states. This, however, is not the case. Realism merely suggests that cooperation is less likely and more difficult to achieve than other theories suggest. Alliances and cooperation are likely to be fleeting and based on material or shared strategic interests rather than on ideas, values, or norms. Some Realists, most notably Mearsheimer, predicted that NATO would dissolve after the Cold War on these grounds.

One of the main impediments to cooperation for Realists has to do with relative versus absolute gains. Imagine that cooperation between two states involves "units of power." A state that values absolute gains would be happy with one unit of power gained from cooperation, even if the other state earned two or more units of power; what matters to the state is that it gains a unit of power. Conversely, a state that emphasizes relative gains would not so readily strike the same deal. Even though the state gains a unit of power, the two or more units of power gained by the other state give that other state a gain in relative power. Such a transaction could be costly under conditions of anarchy and uncertainty. Even though the two states may be on friendly terms in the present, the extra unit(s) of power gained by the other side could be used against one's own state in the future.

Applying Realism to US-Iran Relations

How can Realist international relations theory be applied to US-Iran relations? A person contemplating US-Iran relations might consider the following questions:

• Does human nature preclude peace between the United States and Iran?

• Does the structure of the international state system preclude peace between the US and Iran?

• Are the US and Iran each behaving according to the guidelines and incentives of anarchy and uncertainty?

• Is there a security dilemma between the US and Iran? If so, is there any way to ameliorate this security dilemma?

• Does it matter who is in charge in both the US and Iran?

• Does the internal structure of each government matter: i.e. democracy or dictatorship?

• Is Iran offensively- or defensively-minded? How about the US? Can we even know?

• Does Iran want to become a regional hegemon? If so, is there anything unusual about this?

• Does Iran constitute a sufficient practical threat to American interests and security to warrant the current level of American opposition to Iran?

• Does the US constitute a sufficient practical threat to Iranian interests and security to warrant the current level of Iranian opposition to the US?

• Is there any overlap in the material and strategic interests of the two countries which would allow some amount of practical cooperation?

• Do fears of relative gains stand in the way of cooperation between the US and Iran?

• Are the US and Iran both rational? What does it mean to be "rational," anyway?

This is by no means an exhaustive list of questions that one should ponder. Yet it offers a starting point for those who seek to structure their thinking of US-Iran relations in a theoretical framework. Now, here are some Realist propositions/hypotheses regarding US-Iran relations. Some may seem contradictory; this is not a mistake - it demonstrates that theory is a useful means to structure one's thinking, but is not a magic bullet that provides clear-cut answers. Once again, this is not an exhaustive list:

• The US and Iran will never be able to trust one another because of anarchy, uncertainty, and conflicting material and strategic interests.

• The US and Iran should be able to cooperate on a practical basis because they share some material and strategic interests.

• Iran - quite naturally - seeks regional hegemony.

• The US - quite naturally - opposes hegemony for Iran or for any other state in the Middle East.

• A security dilemma exists between the US and Iran. This security dilemma is perhaps best seen in the continuing showdown over Iran's nuclear program.

• The internal structure and leadership of the two countries do not matter to US-Iran relations. It makes no difference whether the countries are democratic or dictatorial. Nor does it matter whether Bush, Obama, Ahmadinejad, Khatami, or Mousavi occupies the countries' respective presidencies.

• Though the structure of the international state system encourages and discourages certain kinds of behavior, it also provides several policy choices. Though policy decisions will not be totally irrational or vary wildly, it does matter whether Bush or Obama holds office in the US and whether Ayatollah Khamenei or Mahmoud Ahmadinejad wield more decision-making power in Iran than Khatami or Mousavi.