There are times when I would throw the "pre-emptive strike" without hesitation. If I truly believe that this person is about to try to kill me, if they have a weapon drawn and ready, if I am in a rapidly deteriorating multiple opponent situation, etc. etc.
That is not preemptive, that is self-defense.
If you
'truly believe' that you are in imminent danger of being seriously injured or killed, in most jurisdictions, you have the right to defend yourself with force; but the test is that of a 'reasonable and prudent man'. If the police / DA / judge / jury do not think you are a reasonable and prudent man, then you could find yourself convicted of battery at the minimum.
While I'm not going to suggest that the legal system is always correct in assessing what the actual danger was at the moment you decided to act, I will say that our system works pretty well historically. I've dealt with more than a few people who hauled off and slugged someone because they felt they were about to be attacked but could not explain what it was that made them think so, other than
"he just looked like he was fixing to take a swing at me." If that is your criteria, then yes, you are on your own. I would suggest that people acquaint themselves with the laws of self-defense where they live, so at least they will know when they cross the line that they risk prosecution.
And in case you were about to ask - let me answer first. I have explained the same thing to a fair number of drunken numbskulls (I am not accusing of being either one, this is just me talking to the numbskulls of my past) who said
"Oh yeah? Well what was I supposed to do, wait until he punched me first? Screw that!" No, you do not have to wait until you get punched first. You do have to be able to explain rather convincingly why you thought he was about to do so. Like he took a step towards you with his hand raised, or he said
"I'm going to kick your butt," or you had past contact with him and he did assault you, etc. Then I arrest him instead of your sorry butt. If they do something that makes a reasonable and prudent man think an asswhooping was incoming, then by all means, defend yourself. But that is *not* preemptive. You've already been assaulted at that point, even if he hasn't laid a hand on you.
By the way, that also surprises people who get arrested for assault and they say
"What for? I never touched the guy!" Well, if you raise your fist at him and say
"Pucker up, sweetpea!" You just broke the law, so happy trails, brainstem.
Assault is saying
"Ima hit you." Battery is doing it. Both are illegal, and in most jurisdictions, if you fear the assault is about to become a battery, you have the right to defend yourself at that time.