Throughout history, groups have used military force to gain power in the form of territory, plunder, and cheap (slave) labor. Those who did not engage in such activities nevertheless had to maintain military forces to defend against the depredations of others. Today, military force has a multitude of uses beyond aggression and defense against attack. After outlining the nature and different uses of military force in today's world, this chapter focuses on its employment for "peace operations," including third party humanitarian military interventions.