Armed forces may be organized as standing forces (e.g. regular army), which describes a professional army that is engaged in no other profession than preparing for and engaging in warfare. In contrast, there is the citizen army. A citizen army (also known as a militia or reserve army) is only mobilized as needed. Its advantage lies in the fact that it is dramatically less expensive (in terms of wealth, manpower, and opportunity cost) for the organizing society to support. The disadvantage is that such a "citizen's army" is less well trained and organized.

A compromise between the two has a small cadre of professional NCOs (non-commissioned officers) and officers who act as a skeleton for a much larger force. When war comes, this skeleton is filled out with conscripts or reservists (former full-time soldiers who volunteer for a small stipend to occasionally train with the cadre to keep their military skills intact), who form the wartime unit. This balances the pros and cons of each basic organization, and allows the formation of huge armies (in terms of millions of combatants), necessary in modern large scale warfare.

Military science

Military science concerns itself with the study and of the diverse technical, psychological, and practical phenomena that encompass the events that make up warfare, especially armed combat. It strives to be an all-encompassing scientific system that if properly employed, will greatly enhance the practitioner's ability to prevail in an armed conflict with any adversary.