Benefits and Costs
The obvious benefit of any military is in providing protection from foreign armed forces, and from internal conflict. In recent decades standing armies 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, by domestic repression, or simply by supporting the idea that violence (or the threat thereof) is the way to get what one wants.
Military investment in science and technology has brought many side benefits, although some argue that greater benefits would come from targetting the money directly at wanted technologies and basic science.
Over-investment in military forces can drain a society of needed manpower and material, significantly impacting 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 affects the military in a vicious cycle. See the Soviet Union for a typical modern example of this problem.
Transarmament is a recent movement to replace armed forces with nonviolence training and infrastructure.
Armed forces of the world