The armed forces of a country are its government-sponsored defense, fighting forces, and organizations. They exist to further the foreign and domestic policies of their governing body, and to defend that body and the nation it represents from external and internal aggressors. In some countries paramilitary forces are included in a nation's armed forces. Armed force is the use of armed forces to achieve political objectives.
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 three levels study the application of the use of force in order to achieve a desired objective.
See main article: Military organization. In most countries the basis of the armed forces is the military, divided into basic Armed services. However, armed forces can include other paramilitary structures. For example, according to the Lithuanian law on organisation of system of national defence and military service (version actual from November 11, 2004), the Lithuanian armed forces in case of war also include border guards, public police service, parts of Lithuanian Riflemen's Union and guerrillas. 
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.
See also: Deterrence theory.