Full Idea
It is false that by posing a threat to another, one necessarily makes oneself liable to defensive action. A police officer who shoots an active murderer does not thereby by make herself liable to defensive action.
Gist of Idea
You (e.g. a police officer) are not liable to attack just because you pose a threat
Source
Jeff McMahan (Killing in War [2009], 1.2)
Book Reference
McMahan,Jeff: 'Killing in War' [OUP 2009], p.14
A Reaction
This is one of his arguments against the moral equality of combatants. It is not morally OK to shoot all the local soldiers when you unjustly invade a territory. Sounds right to me.
Related Idea
Idea 23597 Just war theory says all and only persons posing a threat are liable to attack [McMahan]