Whether you are interested in the history of war or the contemporary use of it as a tactic in the fight against terrorism, an examination of this complex phenomenon demands careful analysis. Various theories have been developed to explain why nations fight, including those from a philosophical perspective, the anarchic international system, domestic politics, economics, technology, nationalism and terrorism. It is important for the student of war to be aware that, like many social phenomena, definitions of it vary and often the proposed definition masks a particular political or philosophical stance paraded by the author.

This article examines the underlying philosophy behind each of these explanations and evaluates them in light of historical wars, of crises resolved short of war and of controversies that arose around the issue of waging it. In addition, it looks at continuities in the conduct of war, which military personnel must take into account even when they face novel situations.

Resources, or more specifically the control of resources, have always been central to the making of war. This has been as true for early civilisations battling over the availability of food or minerals as it has been for modern militaries engaged in battles over the possession of oil, the uranium needed for nuclear power, the means to sustain an industrialised economy and the ability to maintain international trade.

The second reason for war, argues the Malthusian theory, is that conflict arises from overpopulation and scarcity of resources. The increasing competition for scarce food, energy and raw materials increases tensions, makes it more difficult to find compromise and pushes states towards a state of war. This argument is strengthened by demographic statistics that show the increasing ageing of the world population and the growing proportion of the global population living in poverty, exacerbated by factors such as climate change that lead to lower levels of life expectancy.

Other theories of war argue that a sense of morality and the belief in a just God make it wrong to take innocent lives. This view has been a powerful force throughout history and continues to influence some, mainly western, beliefs about the use of military force.

The final reason for war, argues the neo-classical economists, is that it provides the best way to achieve certain national objectives, particularly the achievement of wealth and power. However, this argument is weakened by the experience of many states that the pursuit of wealth and power leads to corruption, inequality and a lack of political stability. It is also weakened by the evidence of the recent wars that it has not brought prosperity to those who won them. Finally, it is weakened by the fact that fighting wars is ruinously expensive and so countries have powerful incentives to avoid them if possible. This insight is reinforced by game theory. Despite these insights, however, the fact remains that nations do go to war. For this reason, an understanding of why is essential to the study of politics and world affairs.