Throughout human history, war has taken many forms, but it has always involved conflict between two or more opposing parties, and usually results in violence. While there is no one agreed definition of what constitutes a war, some scholars argue that the concept can be defined as a situation in which people fight over territory or resources, usually with serious consequences for those on both sides. Others, however, argue that the term ‘war’ includes non-violent conflicts over beliefs and ideologies.

In the past, the majority of people killed in wars were civilians. This is still the case today. Civilian casualties are the result of indiscriminate weapons that cause massive destruction, such as bombs, artillery, and certain types of improvised explosive devices. Other causes of civilian deaths include malnutrition and disease, sexual violence, displacement, and economic collapse.

The causes of war are complex and vary over time and place, but one important determinant is the willingness of a force to contend, regardless of its odds. This is often referred to as morale, and it has been a key factor in many military defeats as well as remarkable victories. Other factors that contribute to the onset of conflict and war include unresolved regional tensions, a breakdown in political legitimacy or rule of law, illicit economic gain, resource scarcity, and weak or co-opted state institutions.

Conflicts between great powers tend to be more frequent than those between lesser ones, and the former are more likely to be provoked by perceived threats than those of the latter. It is therefore important for the major powers to respect each other’s geopolitical areas of influence and not to overstep them, which can lead to a war of attrition.

It is also important to avoid reliance on technological breakthroughs or’revolutions’ to change the nature of warfare, because such advances may have unexpected and unintended consequences. Moreover, the use of high-tech weapons can make war more costly and less certain to be won by one side or another.

War is a terrible thing, not only because of the physical suffering and psychological trauma it can inflict on people. It also harms education, destroys infrastructure, and leads to mass displacement. Cities in war zones are often severely damaged and suffer from a ‘brain drain’ as the specialist skills of engineers, urban planners, and medical staff are lost to the war zone. In addition, it causes poverty and economic stagnation. The economic costs of a conflict may be so substantial that governments divert money from other essential services to fund it. This can have long-term implications for the health, welfare and prosperity of a country. The article is available as a fully searchable and browsable collection of authoritative references, including articles not yet included in the print edition of Britannica’s Encyclopedia. It also contains a fully searchable version of Merriam-Webster’s Collegiate Dictionary (tenth edition). This is a must-have reference tool for students, researchers, librarians and anyone who needs access to the highest quality and most current scholarship.