The study of the use of Armed Forces is called military science. Broadly speaking, this involves considering offense and defense at three "levels": strategy, operational art, and tactics. All of these areas study the application of the use of force in order to achieve a desired objective.

Organization
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 mobilised 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.

The armed forces in many larger countries are divided into three forces: an army, an air force, and usually a navy (unless geography dictates otherwise). These forces may be solely for the purposes of training and support, or may be completely independent branches responsible for conducting operations independently of other services. Most smaller countries have a single organization that encompasses all armed forces employed by the country in question.

Various countries have a variation on this standard model of three basic forces. Some, following the French model, use four forces, an army, a navy, an air force, and a gendarmerie, all with equal status. Other variations include South Africa (army, navy, air force, military health service), and Egypt (army, navy, air force, air defence.) The United States has five armed forces or services; the US Army, US Navy, US Air Force, US Marine Corps, and the US Coast Guard.

In larger armed forces the culture between the different branches of a countries armed forces can be quite different. It has been said that "a navy and an air force man equipment" whereas "an army equips men".

The state of readiness of a military organisation may be indicated by its DEFCON state (US) or BIKINI state (UK).

Benefits and costs
The obvious benefit to a country in maintaining armed forces is in providing protection from foreign threats, and from internal conflict. In recent decades armed forces personnel have also been used as emergency civil support roles in post-disaster situations. On the other hand they may also harm a society by engaging in counter-productive (or merely unsuccessful) warfare.

Expenditure on science and technology to develop weapons and systems sometimes produces side benefits, although some claim that greater benefits could come from targeting the money directly.

Excessive expenditure on armed forces can drain a society of needed manpower and material, significantly reducing civilian living standards. If continued over a significant period of time, this results in reduced civilian research and development, degrading the society's ability to improve its infrastructure. This lack of development in turn can affect armed forces in a vicious cycle. See North Korea for a typical modern example of this problem.

Transarmament is a recent movement to replace armed forces with nonviolence training and infrastructure.

Source: Wikipedia