A century ago, Prussian general Carl von Clausewitz argued that war is simply “politics by other means.” In other words, we choose to fight over the same things in the same ways—bargaining through violence rather than discussion and compromise. But there are many other ways to bargain, including negotiation and concession. And if we remember that we are all human, we can also avoid war altogether by looking for these alternatives and trying to find compromises that might satisfy both sides.

But despite this, we still have not managed to live without war. In fact, the most common way that we encounter war is as the casualty of a conflict between states or insurgent groups. When this happens, the consequences are atypically devastating for all involved: they are usually long-lasting, and their impact extends far beyond the battlefield.

In addition to the loss of life, a war causes significant damage to the physical and social fabric of a society. The disruption of a country’s economy and the reallocation of resources to war-fighting purposes can be disastrous for people living there, as they struggle to maintain their basic necessities.

The psychological costs of war can be equally profound. The trauma of being exposed to war-related events can lead to a variety of psychosocial sequelae, including somatization (back and stomach pain), sleep disturbances, depression and anxiety disorders, substance misuse and functional disability. These effects can be exacerbated when war is prolonged, as was the case in the 30-year civil war in Sri Lanka between the majority Sinhala and minority Tamil populations.

It is not surprising that, for most of history, war has been one of the most frequent and destructive forces on Earth. It is, after all, the natural consequence of humans’ desire to compete for scarce resources—be they food, water, shelter or land—and to control and dominate those that are not their own.

But there are a number of important differences between competing for and fighting over resources. The first is that the resources of a society are not unlimited. The second is that there are often limits to the capacity of a government to defend itself, even with military expenditures at an appropriate level.

Finally, there is the question of whether a war is justified. The criterion for this has traditionally been that there must be a just cause; a legitimate authority to fight the war; reasonable prospects of success; proportionality—i.e., the morally weighted goods achieved by the war outweigh the morally weighted bads it will cause; and last resort—no other less harmful course is available to achieve the just cause.

But this criteria is problematic because it requires a priori judgments about what counts as a good and what counts as a bad, based on values that are likely to be highly subjective. For example, some commentators have argued that economic progress can count towards the goods of a war, but this would require a sort of lexical priority that is hard to defend.