War is a state of prolonged violent, large-scale conflict involving two or more groups of people. 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 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 if a
peaceable resolution fails, is not sought, or is thwarted, war often results. In
War Before Civilization, Lawrence H. Keeley, a professor at the
University of Illinois, calculates that approximately 90-95% of known societies engaged in at least occasional warfare, and many fought constantly.
[1][2][3][4]
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.