A war is a violent conflict between nations and its causes are complex. A number of theories have attempted to explain the origin, nature, and conduct of war. These include philosophical, political, economic, technological, legal, sociological and psychological approaches. These theories generally have some element of normative significance, since they shape human expectations and determine behaviour.

A key insight from both history and game theory is that nations have powerful incentives to avoid open fighting. This is because war is ruinous for both sides. It is far more expensive than peace and it exposes civilian populations to the risks of terrorism, violence, food shortages, poverty and disease. This makes it much more difficult for leaders to justify the use of force, even if their national interests are threatened, and it makes it much harder to escalate and expand wars once they have begun.

In addition, the costs of war can debilitate nations and thereby undermine their potential for growth. This is particularly important in a world where international trade and capital flows are highly interconnected. When a nation is at war it can be difficult to attract foreign investment, and its exports are likely to suffer. This can lead to a reversal of the gains from trade, with a country losing rather than gaining wealth from its international operations.

Another reason why war is often viewed as inevitable is that, until the advent of modern economics, the world’s wealth was perceived to be finite and the acquisition of more by one nation could only be achieved at the expense of others. This view explains the support for war by some on the far right of the political spectrum, which asserts that it is the natural right of a strong country to take whatever it wants from a weaker one through force.

Other reasons for war include a desire to control territory or resources. This motivation is reflected in the desire for control over the distribution of wealth, and it also motivates the use of force to achieve political goals that cannot be accomplished by peaceful means. For example, some people wish to control the world’s oil supplies, while others are driven by a desire for power and status that can only be obtained through military success.

Historically, war has been a force for change within societies as well as between them. It influences the language that people speak, how places are named, and encourages social changes such as women’s right to work and vote. However, it also has been a major force for destruction and the killing of millions of people.

A key reason for the prevalence of war is that there are still too many countries with unchecked power. This accentuates all five of the motivations for war and it makes leaders more likely to succumb to their idiosyncratic ideologies and biases. It also makes them more likely to engage in deception and miscalculation, and to make promises that they cannot keep.