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 to be large enough to have separate theaters, and the term is not used in the singular. 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 latter is considered by some military historians to be an adjunct of the European Theater.

An American World War II theater of operations

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 US forces were under the joint allied operation command of SHAEF and the administrative command of the American 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 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 control¹.

References

1. Chapter VII: Prewar Army Doctrine for Theater