Realists believe that power is the currency of international politics. Great powers, the main actors in the realist’s account, pay careful attention to how much economic and military power they have relative to each other. It is important not only to have an important amount of power, but also to make sure that no other state roughly shifts the balance of power in its favour. For realists, international politics is synonymous with power politics. They are, however, important differences among realists. The most basic divide is in the answer to the simple but important question: why do states want power? For classical realists  like Hans Morgenthau, the answer is human nature. Practically, everyone is born with a will to power, which effectively means that great powers are led by individuals who are determined to having their state dominate its rivals. Nothing can be done to change that force to be all-powerful  .

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For Neorealists or structural realists, human nature has little to do with why states want power. Instead, it is the structure or architecture of the international system that forces states to pursue power. In a system where there is no higher authority that sits above the great powers, and where there is no guarantee that one will not attack another, it makes good sense for each state to be powerful enough to protect itself in the event it is attacked. In essence, great powers are trapped because they have little choice but to compete with each other for power if they hope to survive  .
Neorealist theories ignore cultural differences among states as well as differences in regime type, mainly because the international system creates the same basic incentives for all great powers. Whether a state is democratic or autocratic matters relatively little for how it acts towards other states. Nor does it matter much who is in charge of conducting a state’s foreign policy. Neorealists treat states as if they were “black boxes”: they are assumed to be alike, save for the fact that some states are more or less powerful than others.  
There is a significant divide between structural realists, which is in the answer to a second question that concerns realists: how much power is enough? Defensive realists like Kenneth Waltz (1979) maintain that it is unwise for states to try to maximize their share of world power, because the system will punish them if they attempt to gain too much power. The pursuit of hegemony, they argue, is especially imprudent. Offensive realists like John Mearsheimer (2001) take the opposite view; they maintain that it makes good strategic sense for states to gain as much power as possible and, if the circumstances are right, to pursue hegemony. The argument is not that conquest or domination is good in itself, but instead that having overwhelming power is the best way to ensure one’s own survival. For classical realists, power is an end in itself; for structural realists, power is a means to an end and the ultimate end is survival.
In a widely discussed essay John Mearsheimer  (1993) use the neorealist argument of Waltz  (1979) and applies it to both the past and future. He says that neorealism has continued importance for explaining international relations: neorealism is a general theory that applies to other historical situations besides that of the Cold War. He also argues that neorealism can be employed to predict the course of international history beyond the Cold War. The question poses is: What would happen if the bipolar system were replaced by a multipolar system?
This question can be justly discussed with the study of the last War in Iraq. Indeed, the Iraq case throws light on the determinants of war, exposing how far decisions are driven by systemic factors. Kenneth Waltz’s ‘defensive realist’ image of systemic constraints shaping a prudent defensive use of power does not appear to correspond to American behaviour. Offensive realism, predicated on the notion that great powers can never have enough power in an insecure world, might seem more relevant, but even this is doubtful: its main proponent, John J Mearsheimer (2001) views hegemony as merely regional and hegemons as acting as offshore balancers outside their own regions. Seeing the Iraq War as going well beyond that, he denied that it was necessary to US security (Mearsheimer and Walt 2003). So, ultimately, How the US invasion in Iraq can be interpreted from neorealists points of view?
First, in order to answer, I will study the works of Waltz and Mearsheimer in identifying their similarities and their differences.
Finally, I will apply their perspective to the US invasion of Iraq.
Defensive Realism versus Offensive Realism: How much power is enough?
The leading contemporary neorealist thinker is undoubtedly Kenneth Waltz (1979). Waltz’s Theory of International Politics (1979) seeks to provide a scientific explanation of the international political system. In Waltz’s view the International relations theory is a neorealist theory that focuses centrally on the structure of the system. Actors are less important because structures compel them to act in certain ways. Structures more or less determine actions.
According to Walts’s neorealist theory, a basic feature of international relations is the decentralized structure of anarchy between sates. States are alike in all basic functional respects in spite of their different cultures or ideologies or constitutions or personnel, they all perform the same basics tasks. But “the structure of the system changes with changes in the distribution of capabilities across the system units”  . In other words, international change occurs when great powers rise and fall and the balance of power shits accordingly. A balance of powers between states can be achieved, but war is always a possibility in an anarchical system. Waltz distinguishes between bipolar systems, such as existed during the Cold War between The United States and the Soviet Union, and multipolar system, such as existed both before and after the Cold War. Waltz believes that bipolar systems are more stable and thus provide a better guarantee of peace and security than multipolar systems. “With only two great powers, both can be expected to act to maintain the system”  . That is because in maintaining the system they are maintaining themselves. According to that view, the Cold War was a period of international stability and peace.
Waltz assumes that the fundamental concern of states is security and survival. He also assumes that the major problem of great power conflict is war, and that the major task of international relations among the great powers is that of peace and security. Nevertheless St ate leaders are prisoners of the structure of the state system and its determinist logic which dictates what they must do in their conduct of foreign policy. There is no room in Waltz’s theory for foreign policymaking that is independent is the structure of the system. Waltz’s image of the role of state leaders in conducting foreign policy comes close to being a mechanical image in which their choices are shaped by the international structural constraints that they face. Thus, Waltz’s neorealist approach does not provide explicit policy guidance to state leaders as they confront the practical problems of world politics. That is presumably because they have little or no choice, owing to the confining international structure in which they must operate. Waltz does address the question of the “management of international affairs”  . Waltz’s argument is at base a determinist theory in which structure dictates policy.
However, just beneath the surface of Waltz’s neorealist text, and occasionally on the surface, there is recognition of the ethical dimension of international politics. For example, he operates with a concept of state sovereignty:”To say that a state is sovereign means that it decides for itself how it will cope with its internal and external problems”  . For Waltz, all states are equal only in a formal-legal sense; they are unequal, often profoundly so, in a substantive or material sense. But that means that a norm of state exists which all states without exception are expected to observe in their relations with each other regardless of their substantive inequalities of power. Waltz also assumes that states are worth fighting for. That, too, indicates that neorealism is imbued with normative values: those of state security and survival.
Waltz operates, as well, with a concept of the national interest: “each states plots the course it thinks will best serve its interests”  . For Waltz, however, the national interest seems to operate like an automatic signal commanding state leaders when and where to move. Waltz sees states as structures that respond to the impersonal constraints and dictates of the international system.
Mearsheimer builds on Waltz’s argument concerning the stability of bipolar systems as compared with multipolar systems  . These two configurations are considered to be the main structural arrangements of power that are possible among independent states. As indicated Waltz claims that bipolar systems are superior to multipolar systems because they provide greater international stability and thus greater peace and security. There are three basics reasons why bipolar systems are more stable and peaceful. First, the number of great-power conflicts is fewer, and that reduces the possibilities of great-power war. Second, it is easier to operate an effective system of deterrence because fewer great powers are involved. Finally because only two powers dominate the system the chances of miscalculation and misadventure are lower. “They are fewer fingers on the trigger”  . In short the two rivals superpowers can keep their eye steadily fixed on each other without the distraction and confusion that would occur if there a larger number of great powers, as was the case prior to 1945 and arguably has been the case since 1990  .
The question Mearsheimer  poses is: What would happen if the bipolar system were replaces by a multipolar system? Mearsheimer  that the demise of the bipolar War order and the emergence of a multipolar world will produce highly undesirable return to the bad old ways of anarchy and instability and even renewed danger of international conflict, crises, and possibly war.
Mearsheimer differs from Waltz whom characterizes as a “defensive realist”: someone who recognizes that states must and seek power in order to be secure and to survive, but who believe that excessive power is counterproductive, because it provokes hostile alliances by other states. For Waltz, it does not make sense, therefore, to strive for excessive power beyond that is necessary for security and survival. Mearsheimer speaks of Waltz’s theory as “defensive realism”.
Mearsheimer agrees with Waltz that anarchy compels states to compete for power. However, he argues that states seek hegemony, that they are ultimately more aggressive that Waltz portrays them as being. The goal for a country, such as United States, is to dominate the entire system, because only in that way could it rest assured that no other state or combination of states would even think about going to war against the United States. In the Western Hemisphere, for example, the United States has long been by far the most powerful state. No other state, Canada, Mexico, Brazil, would even think about threatening or employing armed force against the United States. All major powers strive for that ideal situation. But the planet is too big for global hegemony.”The oceans are huge barriers”. No state would have the necessary power. Mearsheimer therefore argues that states only become the hegemon in their own region of the world  .
Regional hegemons can see to it, however, that there are no other regional hegemons in any other part of the world. They can prevent the emergence and existence of a peer competitor. According to Mearsheimer, thatis what the United States is trying to ensure. That is because a peer competitor might try to interfere in a regional hegemon’s sphere on influence and control. According to Mearsheimer, all states want to become regional hegemons. That is why he refers to his theory as “offensive realism” which rests on the assumptions that great powers “are always searching for opportunities to gain power over their rivals, wit hegemony as their final goal”  . There has always been conflict, there is conflict, and there will be conflict over power. And there is nothing that anyone can do to prevent it. This is why the title of one of his books is The tragedy of Great Power Politics.
In sum, there is disagreement among structural realists about how much power states should aim to control. Offensive realists argue that states should always be looking for opportunities to gain more power and should do so whenever it seems feasible. States should maximize power, and their ultimate goal should be hegemony, because that is the best way to guarantee survival. While defensive realists recognize that the international system creates strong incentives to gain additional increments of power, they maintain that it is strategically foolish to pursue hegemony. That would amount to overexpansion of the worst kind. States, by their account, should not maximize power, but should instead strive for what Kenneth Waltz calls an “appropriate amount of power”  ;
The War in Iraq and the neorealists : a troublesome case
“States operate in a self-help world almost always according to their own self-interest and do not subordinate their interests to the interests of other states, or to the interests of the so-called international community. The reason is simple: it pays to be selfish in a self-help world.”  
The decision made by the Bush administration to invade Iraq in 2003 can both be fit into neorealist theory, while some things also complicate neorealist theory.
Realists believe that power is the controlling force in politics, and especially international politics, and power is defined as the ability to control outcomes. This is a hard thing to prove but the US certainly have shown that it is able to control outcomes, when US forces invaded Iraq without the consent of the UN or most other nations. Other countries like France also tried hard to persuade the US not to invade Iraq, which is also a sign of wanting to control outcomes. However, the US turned out to be a lot more powerful in this case. Mearsheimer’s theory of offensive realism states that the unrelenting pursuit of power means that great powers are inclined to look for opportunities to alter the distribution of world power in their favour. This means that the US, which is by far the greatest power in the West, will always be looking for opportunities to gain even more power at the expense of other states. Neorealist theory can explain the Iraq war in 2003 by saying that the US saw invading Iraq as an opportunity to gain even more power at the expense of foremost Iraq  . The invasion of Iraq can also be seen as a way for the US to prove to the world how powerful it is by not letting the US be taunted by Saddam Hussein, and by defying the opinion of the UN and other states  .
The invasion of Iraq can be analysed as a strategy for gaining power or as a strategy for checking aggressors. By using war as a way of increasing their power, the US can exploit Iraq’s economy for their own benefit and gain power by confiscating natural resources such as oil and foodstuffs  . Also, the US can gain strategic important land by gaining an ally in the new Iraq, which the US has helped rebuild. War has been used as a way of checking aggressors, which is basically a way of preventing other states from gaining power at their own expense. In reality this is done by deterring the aggressor, who in this case is Iraq. When the US did not think that Iraq was deterrable they had no other choice than to invade Iraq, according to offensive realism. This is because the structure of international politics forces states to be aggressive in order to survive. Before and during the 2003 Iraq war the US gave the impression that they were threatened by Iraq, which means that they were trying to deter Iraq, which was an aggressor.
However, looking at the image as being just and democratic that the US clearly wants the world to think of them, it is hard to imagine that they would say anything different from what they did, no matter what their motives were. The conditions of international politics today for countries in the West demands them to be just and democratic – or at least to appear that way – and no state leader would say anything that could not be explained as being just and democratic, because it is the rules of survival in international politics today. This does not mean that states today in the West are only just and democratic, because they can do whatever they want to within reason. It just means that whatever they do will have to be camouflaged as just and democratic.
However, the invasion of Iraq is hard to explain with offensive realism in some ways. According to offensive realism, the central aim of American foreign policy is to be the Hegemon in the Western Hemisphere and have no rival hegemon in Europe or Northeast Asia. How can offensive realism then explain that the US is invading a country in the Middle East, because this region should not interest the US enough to want to invade a country in other regions than Europe and Northeast Asia?  Mearsheimer has trouble seeing why the US have troops in Europe and Northeast Asia, and argues that they should be sent back, and therefore it is even harder to explain why they should have troops in a region in which the US government does not aspire to be a hegemon.
Some of the most prominent realist scholars Mearsheimer and Waltz actually argued against invading Iraq, because they believed that it was unnecessary. All state leaders are rational according to realist theory, which means that Saddam Hussein is also rational although the US government kept arguing that he was irrational and therefore you could not reason with him  . Because realist theory holds that Saddam Hussein, like all state leaders, are rational, he is also deterrable because economic sanctions and threats of massive retaliation will always work on him, and they have in the past, realists argue. Therefore, neorealist theory tells us, that Saddam Hussein is not as big a threat to the US as the US government claimed and there was no reason to invade Iraq. If Saddam Hussein is rational he would acknowledge that Iraq is a weak state, and would never be foolish enough to attack such a powerful state as the US, because it is not wise to attack a state that has nuclear weapons regardless of whether you have WMD or not.
Conclusion
It is impossible to create a theory about international politics, that is bullet-proof, because there will always be an element of unpredictability, which is unavoidable when humans interact. Neorealist theory has been able to predict the US foreign policy because a structural argument: the unipolar system and its instability. However, when the system level ceases to be the major constraint on behaviour, as expected by neorealism, there is much more scope for domestic determinants to drive policy  . Moreover, the war demonstrates the dangers of unipolarity in which the once-benign hegemon becomes malign  . It is no accident that small powers have traditionally put the highest value on international law and the UN and that the world hegemon uniquely deprecates the latter as unwanted constraints on its freedom to do as it pleases; their failure in the Iraq case makes for a less secure world for those at the bottom of the power hierarchy.