War is a reciprocated, armed conflict, between two or more non-congruous entities, aimed at reorganising a subjectively designed, geo-politically desired result. In his book, On War, Prussian military theoretician Carl Von Clausewitz calls war the "continuation of political intercourse, carried on with other means."

War is an interaction in which two or more opposing forces have a “struggle of wills”. The term is also used as a metaphor for non-military conflict, such as in the example of Class war.

War is not necessarily considered to be the same as occupation, murder, or genocide because of the reciprocal nature of the violent struggle, and the organized nature of the units involved.

A civil war is either a dispute between parties within the same nation (such as in the English Civil War), or else a dispute between two nations that were created out of one formerly-united country (such as in the American Civil War). A proxy war is a war that results when two powers use third parties as substitutes for fighting each other directly.

War is also a cultural entity, and its practice is not linked to any single type of political organization or society. Rather, as discussed by John Keegan in his History Of Warfare, war is a universal phenomenon whose form and scope is defined by the society that wages it. The conduct of war extends along a continuum, from the almost universal tribal warfare that began well before recorded human history, to wars between city states, nations, or empires.

In the organised military sense, a group of combatants and their support is called an army on land, a navy at sea, and an air force in the air. Wars may be conducted simultaneously in one or more different theatres. Within each theatre, there may be one or more consecutive military campaigns.

A military campaign includes not only fighting but also intelligence, troop movements, supplies, propaganda, and other components. A period of continuous intense conflict is traditionally called a battle, although this terminology is not always applied to conflicts involving aircraft, missiles or bombs alone, in the absence of ground troops or naval forces. Also many other actions may be undertaken by military forces during a war, this could include weapons research, prison internment, assassination, occupation, and in some cases genocide may occur.

As the strategic and tactical aspects of warfare are always changing, theories and doctrines relating to warfare are often reformulated before, during, and after every major war. Carl Von Clausewitz said, 'Every age had its own kind of war, its own limiting conditions, and its own peculiar preconceptions.'.

War is not limited to the human species. Ants engage in massive intra-species conflicts which might be termed warfare, and chimpanzee packs will engage each other in tribe like warfare. It is theorized that other species also engage in similar behavior, although this is not well documented.

War is a conflict involving the organized use of weapons and physical force by states or other large-scale groups. Warring parties usually hold territory, which they can win or lose; and each has a leading person or organization which can surrender, or collapse, thus ending the war. Wars are usually a series of campaigns between two opposing sides involving a dispute over sovereignty, territory, resources, religion, or ideology. A war to liberate an occupied country is called a "war of liberation"; a war between internal factions within a state is a civil war. Until the end of World War II, participants usually issued formal declarations of war.