The organization that deters states from imposing their political will on other states by force. In the case of America, it provides the muscle necessary to back up the Bill of Rights' guaranteed freedoms. Members of the American military give up most of their civil rights to protect (by force, and with lives, if necessary) the rights of the civilian population at large. A good example would be the free expression of opinions, such as krimson's writeup below.

Many people confuse the military with the military industrial complex of contractors and congressmen--Congress and corporations both work for money. Look at a military pay chart, and you'll realize that nobody in the military is in it for the paycheck.