The use of force, in the context of law enforcement, may be defined as the 'amount of effort required by police to compel compliance by an unwilling subject'. Use of force doctrines can be employed by law enforcement officers and military personnel on guard duty. The aim of such doctrines is to balance the needs of security with ethical concerns for the rights and well-being of intruders or suspects. Injuries to civilians tend to focus attention on self-defense as a justification and, in the event of death, the notion of justifiable homicide. U.S. military personnel on guard duty are given a 'use of force briefing' by the sergeant of the guard before being assigned to their post. For the English law on the use of force in crime prevention, see Self-defence in English law. The Australian position on the use of troops for civil policing is set out by Michael Hood in Calling Out the Troops: Disturbing Trends and Unanswered Questions; compare 'Use of Deadly Force by the South African Police Services Re-visited' by Malebo Keebine-Sibanda and Omphemetse Sibanda. Use of force dates back to the beginning of established law enforcement, with a fear that officers would abuse their power. In today's society this fear still exists and one of the ways to fix this problem is to require police to wear body cameras and to have them turned on during all interactions with civilians. The use of force may be standardized by a use of force continuum, which presents guidelines as to the degree of force appropriate in a given situation. One source identifies five very generalized steps, increasing from least use of force to greatest. It is only one side of the model, as it does not give the levels of subject resistance that merit the corresponding increases in force. Each successive level of force is meant to describe an escalating series of actions an officer may take to resolve a situation, and the level of force used rises only when a lower level of force would be ineffective in dealing with the situation.Typically any style of a use of force continuum will start with officer presence, and end with the use of deadly force.