War is an intensely destructive event that affects everyone involved. Millions can die and cities are destroyed, hurting a country’s economy. This is the primary reason why a nation will only go to war as a last resort. The study of war involves examining all aspects and consequences of the conflict, including its impact on the people who live there. Aside from the obvious physical destruction, many people suffer from psychological trauma, resulting in mental disorders such as post traumatic stress disorder and other conditions. These disorders can have long-term effects, leading to substance abuse and even suicide. In addition, war can lead to a host of other problems, including economic hardship and disillusionment with the government.

The causes of war are a subject of considerable debate, with some theories being more plausible than others. Some scholars believe that human nature and the anarchic international system are important factors, while others focus on domestic politics, terrorism, international economics and technology. Some theorists also suggest that a specific trigger event is required to initiate war, such as a communication of injustice or threat.

In addition, many scholars cite the increasing power of states and the ability to access advanced weapons as changes that have altered the nature of war. Some argue that the increase in the effectiveness of warfare has led to more frequent and prolonged conflicts, whereas others point to the development of political institutions and diplomacy as factors that have mitigated the incidence and duration of war.

Some philosophers also disagree about the role, if any, of morality in the context of war. Some, such as David Hume, claim that the exigencies of war can allow for a lowering of ethical standards and even the omission of justice (Enquiry Concerning the Principles of Morals, sect. 3). Others, such as Michael Walzer in his essay “Just and Unjust Wars,” argue that certain moral constraints remain applicable to any war, irrespective of its cause.

Regardless of the precise causes of war, all theories of it share one basic assumption: that human beings have an innate desire to defend themselves against attack and to acquire the means necessary to live. As such, most of the theories of war begin with a discussion of the rationality of that desire and then proceed to examine various explanations of the conditions that create such a rationality.

While the majority of wars are between states, a significant percentage of conflicts are non-state, either involving a state’s proxy forces or non-declared actions such as guerrilla warfare. The study of non-state wars, such as terrorism and the war in Afghanistan, requires an additional level of analysis and understanding. Moreover, the study of non-state wars often highlights the necessity of examining other causes of war, such as poverty and economic disparity. These issues may not be as easily addressed by military intervention as those of states, but are nonetheless critical to a full analysis of war and its underlying motivations.