The purpose of Book V is to examine the armed forces. The key areas of study include their numbers, organization, supplies and maintenance, their state outside of combat, and their relationship with the country and the territory. Clausewitz explores these categories as the “necessary conditions of fighting” (155).

The theater of war describes “such a portion of the space over which war prevails as has its boundaries protected” by using buildings or the natural elements of the terrain (155). An army is “the mass of troops in the same Theater of War” which is organized and led by a chief command (155). A theater of war and an army “mutually include each other” (156). In turn, a campaign is “the sum of all military events which happen in all Theaters of War in one year” (156). Occasionally, campaigns are interrupted by winter. They can also last longer than a year based on the logical conclusion.

The means of modern warfare presuppose that “the superiority in numbers becomes every day more decisive,” as does its “principle of assembling” (156). Numbers also matter because most European armies are comparable in organization, training, and equipment, so “[t]he character of modern battles is the result of this state of equality” (157).