A war is a conflict between two or more groups that involve large numbers of individuals. Wars may be prosecuted simultaneously in one or more theatres of war. Within each theatre, 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 sovereignty, territory, resources, religion, or ideology and a peaceable resolution is not sought, fails, or is thwarted, then 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. A declaration of war is not normally made in internal wars.

source: http://en.wikipedia.org