I would say (my opinion) that a 'pre-emptive' strike is never valid. By definition, it is acting not on something which has actually been done but on a fear or assumption that it might happen. This is the basis on which I opposed the attack on Iraq, but supported that on the Falklands -- the latter was in response to an initiation of force by another country, whereas with Iraq there was merely a "feeling that they might do something".

The same applies on a personal level, as far as I'm concerned the person who initiates force is assumed to be in the wrong unless they can provide very good reasons (which will satisfy a jury or other collection of people representative of the society) why it was necessary. For instance, a mother using force to restrain a child which is about to (or likely to) run out into the road in front of a car is reasonable (and for that matter if I restrained you from doing the same it would be reasonable), but use of excessive force so that it caused damage might not.

This in general distinguishes a 'need' from a 'want'. I could say "I need a new guitar", but if I attacked someone to get the money to buy it I would rightly be condemned by almost all other people, because my actions were not proportionate to the level of need, and they would say (again correctly in my view) that no, I just wanted the guitar. If it were a loaf of bread to keep myself (or my family) from starving they might look on that as more justified (although probably not enough to save me from punishment).