The student of war should be careful in examining definitions of it, for like any social phenomenon it has many different etiologies. One way to avoid such traps is to adopt a working definition of war that does not necessarily mask a particular political or philosophical stance paraded by its proposer.
This definition has the advantage of permitting a broader view of its causes than is possible with a more restrictive or reductive definition that focuses on the state as the only source of war. It allows for a consideration of the causes of war within non-state entities, such as guerilla movements or tribal conflicts that have no state backing and appear spontaneous. It also permits a consideration of conflict that may not be explicitly or formally declared, such as the armed interference in civil strife by a foreign power in a sovereign nation.
It is often argued that the desire to fight is a fundamental human trait, shared by all species. It is argued that the human brain is wired to want to achieve goals by any means necessary. It is argued that humans have a genetic makeup to be natural killers, although this claim is disputed by those who argue that there are no scientific studies of the mental and physical capabilities of women, who in fact seem to possess a number of capacities to kill with equal effectiveness as men.
Another line of argument, sometimes referred to as cultural determinism, is that the desire for conflict is due to the existence of certain cultural institutions and the need for human groups to adapt to their environments. This argument is contested as well, with differing opinions as to the nature or possibility of culture change and as to whether the desire for conflict will always prevail, regardless of whether cultures are conditioned to it.
Some critics of Clausewitz, such as Brian Holden Reid, suggest that he fails to take into account some of the fundamental drivers of war. It is argued that he looks at war only from the demand side, as something that states require for their purposes and fails to consider why people, including states, may see war as valuable in itself.
In this view, some wars have very ambitious goals that require massive effort and cause great destruction. Other wars have modest goals and show little ‘hostile spirit’ and are therefore more limited in their impact. The tendency for ambitious wars to escalate is viewed as a fundamental feature of warfare and an intrinsic part of its causation. The desire for power and the pursuit of glory are also considered to be important factors driving some to seek war. Other motives, such as resentment of past injustices, are also seen as potentially driving some to seek war. However, these are not universal motivators for a war. Some states do not engage in aggression until they have amassed sufficient material resources for a war, and even then they may not be prepared to use them unless they can be assured of winning.