Classical realism and structural realism are both significant theories in the realm of international affairs and share a number of key assumptions about the state and the international system. These assumptions mainly center around the characterization of an anarchic international system in which trust is scarce and self-help is the predominant strategy of all states. However, outside of their core assumptions, they significantly diverge from each other when attempting to predict the actions of states, theorizing the most stable polarity of the international system, and how they reach their conclusions. Although, when analyzing each of these theories, it is important to understand that classical realism frequently acts more as a critique of structural realism rather than its own theory although it has been used as an independent theory more frequently in recent times, and that structural realism builds upon the basis of classical realism. Critiques of structural realism by classical realists are best understood in the context of real-life examples, such as the Cold War, in which variables such as levels of analysis are considered by classical realists, while structural realists stick purely to their rational actor model. Overall, both theories are able to provide unique insight into the workings of the international system and their usage can result in significantly different consequences for policymakers, even while sharing a number of key assumptions.
The basis of realist theory as a whole stem from its key assumptions of the international system: egoism and anarchy. Egoism in regards to human nature holds that humans inherently only look out for their own self-interest, while anarchy refers to the belief that the state is the highest authority in the international system and there is no higher authority that can legitimately challenge the sovereignty of states. Thus, when characterizing the international system as a whole in both classical and structural realism, the international system inherently lacks trust as all states only look out for their own motives, which reinforces the belief that all states must act in a self-helping manner in order to survive. The combination of these key assumptions leads to both theories' belief that the international system is inherently bent towards conflict between states, rather than peace. However, neither theory is defined by constant violence, but rather the belief that conflict cannot be completely unavoidable and thus all states must do what is best for their own interests in regards to security, sovereignty, and power. This ultimately leads to both theories assuming that power in the international system is of the highest importance, as the accumulation of power can lead to a greater probability of safety in an anarchic, self-help environment. Similarities between the two theories also go beyond key assumptions, as they both share an emphasis on the importance of balance and power and the security dilemma, however they each draw different conclusions from these ideas. Lastly, the greatest similarity between classical realism and structural realism is their focus on the world as it is, rather than how the world should look like. In both theories, to successfully act within the international system is to understand the inherent characteristics of the system and any attempt to try and forcefully shape the system will be undermined by the key assumptions of egoism and anarchy.
Although classical realism and structural realism share a number of similarities in regards to key assumptions, the conclusions that they draw from the international system are significantly different due to their difference in view of where these assumptions are derived from. Classical realism believes that the origin of egoism and anarchy in the international system stems from human nature. This can be traced back to Thomas Hobbes’ book Leviathan, which states that human nature is defined by its selfishness and that government is necessary in order to avoid anarchy and chaos. From this origin, classical realism attempts to analyze and predict state actions on levels of analysis. The two most important levels of analysis are the domestic and system level, which aim to encompass history, politics, and individual characteristics of leaders and actors in the potential decisions that a state may make.
This is in stark contrast to structural realism, which attempts to avoid human nature and levels of analysis and opts for analysis based on the rational actor model. For structural realists, states are assumed to be rational actors that make cost benefit calculations in regards to the international system. This stems from the desire of structural realists to minimize the amount of non-quantifiable variables in relation to international relations and thus provide a more reliable method of predicting/analyzing state behavior. The rational actor model is best related to the Prisoner’s Dilemma, which is a hypothetical situation in which two criminals are not given the option to communicate when deciding whether or not to testify against each other. In this situation, the suboptimal outcome of both prisoners testifying against each other is the most likely outcome, even when cooperating with each other would end in a better outcome. This calculation comes from the fact that if one prisoner were to testify while the other does not, then the prisoner that opted not to testify would receive the worst possible outcome, thus incentivizing both prisoners to testify against each other. For structural realists, this acts as an accurate representation of the international system as a self-help environment, and thus the international system is inherently skewed towards suboptimal outcomes, purely because of each state's cost benefit analysis.
In terms of classical realism and structural realism, ideas such as the balance of power and the security dilemma are prominent ideas that can be used to analyze the international system, however they each take very different stances on how they should be analyzed. For classical realists, the balance of power is best understood in the context of great powers, hubristic overreach, and the equilibrium state. Great powers are generally understood to be states with a great deal of influence over other states, whether this influence be economic, political, or militarized. According to classical realism, the international system has an equilibrium state that is not defined by violence but is violence prone. This violence is usually due to the hubristic overreach of great powers. Great powers are allocated a certain extent of leeway in their actions in the international system due to their advantage of hard power, however, if great powers overextend themselves and engage in acts of hubris, which they often due as it is inherent within human nature, then other states will step in to stop such actions. This is best exemplified by Athenian hubris in their invasion of Melos, which threw off the equilibrium state of the international system and ultimately led to its downfall. A classical realist would attribute this mistake to the inherent flaws of human nature in being selfish and hubristic, however, structural realists would view this in the context of a rational actor model and state that Athens engaged in suboptimal behavior due to cost benefit analysis heavily influenced by the self-help environment.
Structural realists prefer not to analyze events in terms of human nature and thus their view of the balance of power is quite different from that of a classical realists’. An offensive structural realist would believe that a bipolar system would bring the most stability to the international system as states will always attempt to maximize power in relation to their opponents as international relations is a zero-sum game and two great powers focusing on each other would allow for better choices to be made than if numerous great powers were to coexist or if a hegemony were to exist. However, a defensive realist would suggest that a multipolar system would bring the most stability as states have an understanding that a buildup of arms is seen as aggressive by other states, and so other states are much more willing to engage in conflict to preserve balance. Both of these differ from classical realism as it offers no prediction on what polarity of the international system is most stable, but rather it assumes and accepts that there is an equilibrium state, however, this would more closely resemble defensive structural realism rather than offensive structural realism.
In regards to classical realism’s critiques of structural realism, it is most obvious when analyzing specific events, such as the Cold war and the Peloponnesian war. The Cold war serves as a classic example of the security dilemma as the USSR and the U.S. were both building up conventional and nuclear arms in response to each other’s supposed aggression, leading to a seemingly uncontrollable arms race. As neither state could trust the intentions of the other due to the inherent self-help structure of the international system, each new advancement in hard power resulted in a greater advancement of hard power by the other state in the name of security. The continuous build up by both states led to the U.S. attempt at developing the strategic defense initiative which would supposedly make nuclear armaments obsolete if Reagan were to give it to the USSR. Both classical realists and structural realists in this situation would have supported the avoidance of such a deal as other states in the international system cannot be trusted, however, where this lack of trust comes from is significantly different. A structural realist would say that the USSR could not trust the U.S. to provide them with the weapons developed by the strategic defense initiative as it would not be a rationally sound decision for the U.S. to cooperate with the USSR and give up their hard power advantage, and thus the U.S. was bound to defect from the deal if the USSR were to even accept it. Even if the U.S. were to actually give the weapons developed from this program to the USSR, there would have to have been a caveat in which the U.S. gains an advantage as the intentions of other states can never be trusted in a self-help environment. A classical realist would come to the same conclusion of avoiding the deal, but mainly due to their belief that human nature and history would have played a role in either state defecting from the deal.
Classical realism’s emphasis on history and domestic politics is one of its most common critiques of structural realism. This critique is most obvious when analyzing the resolution of the Cuban missile crisis and other great power conflicts throughout history. As the U.S. was able to win the battle of escalation dominance in regards to the USSR stationing and ultimate removal of nuclear armaments and troops in Cuba, structural realists would state that this resolution came from the rational actor model and the USSR had to back down as the costs of nuclear warfare were too great to be worth such conflict. However, classical realism would point towards the historical evidence of great power conflicts leading towards conflict and would explain the resolution of the Cuban missile crisis more on the domestic or even individual level. Such critiques would be that structural realism did not consider the lack of accurate information that the U.S. had in regards to the amount of USSR troops stationed in Cuba and how much influence Castro actually had in utilizing the nuclear armaments. Without such crucial information, classical realism could conclude that this was blind luck rather than a cost benefit analysis as such analysis cannot take place when such significant variables are not fully understood. Classical realism would also point out that the Cold war is one of the only bipolar systems in history to have ever not devolved into conflict due to hubristic overreach, and would criticize offensive realism for its support of such a balance of power. However, it would have also criticized structural realists for not analyzing the crisis in regards to domestic politics, as domestic politics is seen as a source of limits on great powers in order to avoid the Thucydides trap. This belief in classical realism is derived from the Peloponnesian war and is referred to as the Thucydides trap. The Thucydides trap states that "when one great power threatens to displace another, war is almost always the result" and is derived from the fall of Athens in the face of the rise of Sparta as a great power. However, this trap can be avoided only by well-functioning domestic politics as leaders can easily fall for hubristic overreach unless they are held back by political mechanisms and the will of the people.
Lastly, the differences between these two theories may result in monumental consequences for policy makers depending on the situation. For example, a policy maker who believes in a theory of structural offensive realism may attempt to upset the equilibrium state in the international system in order to achieve a more stable, bipolar balance of power. However, if other states’ leaders and policymakers are of the classical realist camp, they would view this as hubristic overreach rather than a step towards stability and possibly initiate conflict in order to sustain the equilibrium state of power. However, an offensive structural realist policymaker most likely would not have attempted to engage in conflict as there may be too great of a cost in attempting to stop a state from achieving great power status, which would incentivize buck passing. On the other hand, a defensive structural realist may enter conflict to a certain utility maximizing extent, which would differ from a classical realist who would enter into conflict until a new balance of power is established or until norms or domestic policies force policymakers to stop the conflict. Overall, the seemingly subtle difference between classical realism and structural realism may very well be the difference between appeasement and total war. For example, had Neville Chamberlain been an offensive realist, he most certainly would not have appeased Hitler’s actions at Munich and sign the Munich pact, but rather may have opted to involve the British and possibly even the French into a conflict with the Germans much earlier than it had actually started in an effort to preserve the balance of power established at the end of World War I. The same could also have been said for Hitler himself, as if he were a defensive structural realist, he most likely would not have accounted for the fact that the British certainly were going to appease him due to domestic predilection towards peace rather than another conflict, and he most likely would not have attempted to reinstate the Luftwaffe and rearm the German military as he would have believed that other states would see this as aggressive and possibly attempt to counter him immediately. Thus, history may have looked very different if the theories supported by individual policy makers had been either a variant of structural realism or classical realism.
Overall, it must be understood that both structural realism and classical realism are significant methods of analysis and prediction in the realm of international relations. As they both fall under the umbrella of realist theory, there are a number of key assumptions and similarities they share, but also a significant difference in regards to motives, origins, and outcomes when actually implemented by policymakers and theorists. Although they are not the only tools available to understand the international system, when attempting to view decisions in the moment with respect to how the international system is actively structured, they prove to be very useful in understanding the actions of other states.