Wikipedia
Defensivism is a philosophical standpoint related in spirit to the Non-aggression principle. It is also a midway point between other combat-based philosophies, that of Just War and Pacifism.
The standpoint of Defensivism is that only Defensive actions are moral. You may move to someone's aid as long as they are in immediate danger, or protect yourself from immediate harm.
To a country, it holds that a military force may never leave its home country's own borders except to move to the aid of someone else (and in such a situation, that they may only protect sites from harm that they are specifically invited to, with no aggressive action taken whatsoever).
Any form of preemptive strike, any type of capture, revenge, or any situation where one would fire the first actual shot or throw the first actual blow is against the Defensivist standpoint. Also, any form of combative action must cease the moment the opposition stops fighting, withdraws, gives up, or ceases aggressive action.
Generally, Defensivism only allows the taking of life when the life that would be taken is actively threatening another life.