War is a state of organized collective conflict and hostility between states or groups of people. It is also a term that may be used more generally to refer to any active state of hostility or conflict, whether it be in the form of fighting between military units, militias, or even within families and communities. There are several schools of thought on the causes and legitimacy of war. Some scholars believe that human nature inevitably makes us warlike, while others argue that there are many ways to prevent war, including limiting the role of the state and fostering group cohesion.

One view of the cause of war is that it stems from a natural human desire for protection or acquisition of critical resources such as food, water, or land. This argument has led to the development of a number of different strategies for the prevention of war, including de-escalating conflict through negotiations and arbitration.

Other scholars are more inclined to attribute the origins of war to state-based political institutions, especially those with a strong sense of power and responsibility. These theories rely on concepts of Humean democracy and the belief that the government is ultimately accountable to its citizens. Consequently, they hold the political leadership responsible for initiating war and determining its duration and scope.

These arguments are often supplemented by considerations of morality, particularly the sanctity of human life. This has given rise to a large body of literature on the concept of just war. Its adherents argue that the legitimacy of war hinges on its justification, which can be achieved by examining the rationale for engaging in violence against an enemy and ensuring that the means are proportionate to the threat to the state.

The justification of war is a contentious issue, with some academics advocating for the adoption of a sliding scale that allows for the abandonment of certain ethical constraints when facing graver threats to the body politic. Others, such as David Hume, argue that the sanctity of human life should not be compromised, even in the case of war.

There are many costs to war, both for the civilian population and the armed forces. For the former, it can lead to loss of income and livelihood, which can be very difficult to recover from, as well as long-term health issues such as PTSD. In addition, war can impose restrictions on international trade, leading to economic stagnation and increasing levels of poverty.

The armed forces incur considerable costs in terms of training, equipment, and supplies. This diverts funds from other economic development initiatives and can increase unemployment rates among veterans upon their return home. Moreover, it can also impact the ability of governments to provide educational services for children and healthcare for their citizens. Lastly, it can have a significant negative impact on the environment by destroying buildings and contaminating water supplies. Moreover, it can lead to refugees, whose lives are often fraught with instability and uncertainty.