War is a prolonged state of violent, large-scale conflict involving two or more groups of people. When and how war originated in humans is a highly controversial topic. Some believe it began only about five millennia ago, with the rise of the first states; afterwards war "spread to peaceful hunter-gatherers and agriculturalists" (Otterbein 2004: 31-32). Others (Azar Gat 2006 36-37) argue that war originated in the hunter-gatherer past. Keith Henson proposes that future outlook, memes and behavioral switches account for the origin of wars. Often two or more different leaders or governing bodies have a disagreement and engage other individuals to fight for them - even if those fighting have no interest vested in the issues fought over. The original cause of war is not always known. Wars may be prosecuted simultaneously in one or more different theaters. Within each theater, there may be one or more consecutive military campaigns. Individual actions of war within a specific campaign are traditionally called battles, although this terminology is not always applied to contentions in modernity involving aircraft, missiles or bombs alone in the absence of ground troops or naval forces.
The factors leading to war are often complicated and due to a range of issues. Where disputes arise over issues such as territory, sovereignty, resource, or ideology, and a peaceable resolution fails, is not sought, or is thwarted, war often results.
A war may begin following an official declaration of war in the case of international war, although this has not always been observed either historically or currently, nor in the case of civil wars. A declaration of war is not normally made in internal wars.