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In warfare, a theater or theatre is normally used to define a specific geographic area within which armed conflict occurs. A war would have to occur over a large portion of the globe in order to be considered large enough to have separate theaters, and the term is not used for wars that play out in a single region. Typically, each theater would be distinct and separate from other theaters. Very often, the delineation occurs along continental boundaries or in separate oceans. Typically, in order to be considered multiple theaters in a single conflict, at least one of the nations involved must be participating in multiple theaters; without this, each area is considered a separate war.

The best (but not first) example of a war with several large and distinct theaters is World War II. This war had at least three separate theaters: European, Pacific, and African, though the last is considered by some military historians to be an adjunct of the European Theater. The Eastern Front may be considered separate from the West European Theater by some.

The term seems to have been coined by Carl von Clausewitz in his book "On War"^[1]. Specifically in his book he defines the term thus: "This term denotes properly such a portion of the space over which war prevails as has its boundaries protected, and thus possesses a kind of independence. This protection may consist in fortresses, or important natural obstacles presented by the country, or even in its being separated by a considerable distance from the rest of the space embraced in the war.—Such a portion is not a mere piece of the whole, but a small whole complete in itself; and consequently it is more or less in such a condition that changes which take place at other points in the seat of war have only an indirect and no direct influence upon it. To give an adequate idea of this, we may suppose that on this portion an advance is made, whilst in another quarter a retreat is taking place, or that upon the one an army is acting defensively, whilst an offensive is being carried on upon the other. Such a clearly defined idea as this is not capable of universal application; it is here used merely to indicate the line of distinction."

Chart 12.- Typical organization of a theater of operations as envisaged by War Department Doctrine, 1940

An American theater of operations was an administrative term for a theater which had both an operational and an administrative command. For example, in the European Theater of Operations, U.S. forces were under the joint allied operation command of Supreme Headquarters Allied Expeditionary Force (SHAEF) and the administrative command of the "European Theater of Operations, United States Army" (ETOUSA); but in the China Burma India Theater, American forces only had an administrative command as the operational command of ground troops was (theoretically) through the British 11th Army Group which reported to the joint allied command Southeast Asia Command (SEAC).

The term "theater of operations" was defined in the [American] field manuals as the land and sea areas to be invaded or defended, including areas necessary for administrative activities incident to the military operations (chart 12). In accordance ]]with the experience of World War I, it was usually conceived of as a large land mass over which continuous operations would take place and was divided into two chief areas-the combat zone, or the area of active fighting, and the communications zone, or area required for administration of the theater. As the armies advanced, both these zones and the areas into which they were divided would shift forward to new geographic areas of control^[2].

The concept of theater warfare has been used in several wars, and often serves as a linchpin of strategy for one or more sides.

For example in the American Civil War, one key strategy of the North was to attack the South in both the Western and Eastern theaters, in order to use the North's greater resources to force the South to over-extend its forces. Thus, Ulysses Grant's capture of Vicksburg, which split the South in half by capturing the Mississippi River, had a major effect on General Robert E. Lee's eastern operations, by reducing the supplies Lee received.

When Grant became commander of the entire Union Army, he ordered generals in widespread theaters to coordinate their operations in order to impede the South from transferring troops to various places.

Wars of the empires of the 15th-19th centuries often involved fighting in widely-separated regions, as imperialist powers acquired colonialist possessions throughout the world, on various continents and regions; these could be considered to be multi-theater wars.

In World War I, several British leaders including Winston Churchill suggested that Great Britain and the Allied Powers expand their operations in the Middle Eastern theater, to place increased pressure on the Ottoman Empire and other Central Powers.

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• Theatre of Operations in Information Operations

1. ^ Carl Von Clausewitz, On War
2. ^ Chapter VII: Prewar Army Doctrine for Theater