What might constitute an appropriate use of force by law enforcement officers varies from situation to situation and country to country. In the US, police officers receive guidelines and training on using force from the individual agencies by which they are employed. However, there appear to be no universal standards that govern when to use force or how much to use. Instead, force is generally deemed proper if it was necessary and not excessive.
One thing that all law enforcement officers in most of the world share is that they are legitimized by their governments to use appropriate force. They can face situations involving potential harm to themselves or others with little time in which to make their decisions. Most guidelines are based on the experience of the particular police agency, the function the officers are performing, and any state or federal guidance available to them.
Most police officers, in the US and other countries, operate within a continuum of force and must decide, sometimes quickly, what level of force to use. The continuum ranges from mere officer presence to diffuse a situation, to the use of lethal force. In between are verbal commands, the use of physical restraint, batons, chemical sprays, taser guns, and less-than-lethal force. The use of each must be necessary and proportionate to the circumstances.
There is some international consensus on approaches to the employment of force by police personnel. A United Nations Conference adopted the Basic Principles on the Use of Force by Law Enforcement Officers. Principles include the development by government agencies of regulations regarding the use of force and weapons. Non-lethal and incapacitating weapons should be used as far as possible before using lethal force. Any type of force should be employed with restraint, administered in such a way as to minimize injuries, and accompanied as soon as possible with necessary medical care.
In most jurisdictions, force is considered improper and unjustified in any case where it is not necessary to achieve compliance with a lawful police directive. Force becomes excessive when more of it is used than necessary to accomplish a legitimate police purpose. Use of lethal force requires a threat of serious physical harm to the officers or civilians.
Lethal use of force is considered unnecessary when less intrusive means are realistically available to the police. US courts deciding cases of improper use of lethal force look at whether there was a threat of serious physical harm and the necessity for using lethal force to prevent it. Courts also recognize that there may be extreme situations when there is no opportunity to consider other alternatives to lethal force.