The study of war has always attracted a lot of attention. Over time, many theories have been proposed and refined to understand the causes of international war. Some of these theories rely on cultural and psychological nature of individual leaders or man in general. Other theories focus on the decision making process of the regime or domestic politics to describe the causes of war (Reiter 2003). However, the two most prominent theories or perspectives on the causes of International War and International relations areand. These theories consider the state as single rational actor. In other words both are system level theories that consider the state as the main actor in the International system.This article is the first part of a two-part series which will initially describe the Realist and Liberal perspectives on war and international relations and what causes war according to each theory. It will then discuss these perspectives with examples of wars involving major and minor power and how the Realist perspective – particularly neorealist views like offensive realism – is better placed to explain these. It will also briefly discuss how liberal peace theory or the absence of war can be explained by alternative realist explanations as well. Next, it will discuss how nuclear weapons may have fundamentally changed the dynamics of great power war in the modern era. Finally, we will conclude by briefly discussing how states still continue to play the balance of power game and are increasingly more likely to balance threat and not just power as they continue to try to maximise their relative power in international system.The main purpose of this two-part series article is to allow new and existing members of DFI to appreciate the current themes in academeic and policy making spheres and develop a critical ability to analyse and think about world affair and apply these concepts appropiatelyhas been historically the dominant paradigm to explain state behaviour and causes of war. It basically holds a pessimistic view of International politics. According to the neorealist perspective, state behaviour is primarily driven by survival in the International system. Therefore, the state tries to survive by maximizing its power and trying to change the balance of power in its favour. Power of a state can be defined in many ways but usually consists of military force a state possesses as well as latent power in terms of population and wealth or its economic power. Neorealists like Mearsheimer (2001) have explained that the very structure and anarchic nature of the international system forces the state to behave in a manner to ensure its survival. Mearsheimer’s Offensive realism theory assumes that states are rational actors that exist in an anarchic world, which simply refers to the lack of a higher authority on top of nation-states system. This means that there is no foolproof way to know what the intentions of other states are in the system. This leads to a case of the “security dilemma” where a defensive action taken by a state can be perceived by another as an act threatening its survival (Jervis 1978). Hence the rational response for states is to increase its power and capability to ensure its survival. These actions will lead to what is known as “balancing” behaviour by states.In this theory, war is one of the strategies adopted by great powers to maximize their power and ensure their survival. Great powers may make miscalculations where they fear other states gaining power for their own survival which may lead to a countermove by a rival state moving them on a path towards war. In other cases, only the threat of force or “blackmail” may be used to maximise power without actual war taking place. Other states may encourage war between rival states to reduce power of both states to eventually increase their own relative power in a case of “bloodletting”. (Mearsheimer 2001)The polarity i.e. the number of great powers in the system also plays a major role in the possibility of war. This is where Mearsheimer (2001) explains that a balanced bipolarity with only two great powers would be the most stable with very low possibility of these states going to war with each other. The Cold war where the US and USSR maintained a bipolar system is an example of this. On the other hand an unbalanced multi-polar system is likely to see multiple wars both between major and minor powers as well as between major powers. The example for this is the state of great powers in Europe before WWI and WWII.The other competing view on International relations and war is. This is essentially based on an optimistic view of International politics. Although there are many sub-theories under this paradigm, the main idea is that the security dilemma that most states suffer from in an anarchic system can be alleviated under some guiding principles. Russet (2001) describes threeprinciples that form the main legs of the liberal peace theory.These principles are the type of regime of the state and whether it is democratic or not; the interdependence of free trade between states and participation in international institutions that can foster co-operation and acceptable norms. The relative power of a state is not considered the overriding reason for the state’s behaviour and it believes that states can alter their behaviour to rise above “power politics”. The three Kantian principles when applied, can create a virtuous circle where increasing democracy, interdependence in trade and increasing co-operation in international institutions results in making war highly unlikely (Russet 2001). This would then foster a sense of collective identity and norms that would further result into a creation of security communities where eventually even the thought of going to war would cease to exist (Deutch 1968).According to this theory, war occurs when these Kantian principles do not exist. So for instance, states that are autocratic are more likely to be involved in war. The lack of interdependence through trade also reduces the incentive for states to maintain peaceful relations. Finally the lack international institutions do not help in alleviating the anarchic nature of the international system and this increases the chances of misperception and miscommunication that may result in war. In this theory, relative power of states and number of great powers or polarity does not play a major role in war and peace.Both perspectives can provide powerful explanations to the causes of war. The most powerful support to the liberal perspective is the consistent decline in war and great power conflict since the end of WWII and subsequently after the Cold War. The empirical data shows that conflicts have been declining consistently as liberal Kantian principles have been adopted by increasing number of states (Gleditsch 2008). However, in my view, neo-realist theories like offensive realism provide much better and more consistent explanations on International war. In fact, many examples of liberal peace can be explained by a neorealist perspective too we shall see in part II of this article.