ARTICLE XVII. Of the Theater of Operations. The theater of a war comprises all the territory upon which the parties may assail each other, whether it belong to themselves, their allies, or to weaker states who may be drawn into the war through fear or interest. When the war is also maritime, the theater may embrace both hemispheres,--as has happened in contests between France and England since the time of Louis XIV. The theater of a war may thus be undefined, and must, not be confounded with the theater of operations of one or the other army. The theater of a continental war between France and Austria may be confined to Italy, or may, in addition, comprise Germany if the German States take part therein. Armies may act in concert or separately: in the first case the whole theater of operations may be considered as a single field upon which strategy directs the armies for the attainment of a definite end. In the second case each army will have its own independent theater of operations. The _theater of operations_ of an army embraces all the territory it may desire to invade and all that it may be necessary to defend. If the army operates independently, it should not attempt any maneuver beyond its own theater, (though it should leave it if it be in danger of being surrounded,) since the supposition is that no concert of action has been arranged with the armies operating on the other fields. If, on the contrary, there be concert of action, the theater of operations of each army taken singly is but a zone of operations of the general field, occupied by the masses for the attainment of a common object. Independently of its topographical features, each theater upon which one or more armies operate is composed, for both parties, as follows:-- 1. Of a fixed base of operations. 2. Of a principal objective point. 3. Of fronts of operations, strategic fronts, and lines of defense. 4. Of zones and lines of operations. 5. Of temporary strategic lines and lines of communications. 6. Of natural or artificial obstacles to be overcome or to oppose to the enemy. 7. Of geographical strategic points, whose occupation is important, either for the offensive or defensive. 8. Of accidental intermediate bases of operations between the objective point and the primary base. 9. Of points of refuge in case of reverse. For illustration, let us suppose the case of France invading Austria with two or three armies, to be concentrated under one commander, and starting from Mayence, from the Upper Rhine, from Savoy or the Maritime Alps, respectively. The section of country which each of these armies traverses may be considered as a zone of the general field of operations. But if the army of Italy goes but to the Adige without concerted action with the army of the Rhine, then what was before but a zone becomes for that army a theater of operations.