As to self defense. I can tell you what every police officer has told me who has responded to a situation I was involved.
"You do not have the right to hit them first. You did not know they might have stopped."
Or
"You do not have the right nor the responsibility to defend someone else. After they hit the person you can get involved if their life is at danger. But before hand I can tell you that YOU cannot make that call."
While Striking first even on someone swinging on you, or blocking or passing or steeping out of the way and striking means you have made first contact. The law could state that you were able to pass or block, why not continue until the other guy decided to stop.
So understand no matter where you are, no matter what you do, the situation will be taken in the worse possible light. At least that is my experience.
Rich, I take your point and experience that first strike may put you in a complicated legal position but the concept of a pre-emptive self defence strike is a defence under criminal law in many western jurisdictions and in some US ones I have looked at.
If you can reasonably determine that you or another person's life or well-being are in danger, then striking first is a recognisable action of defence. The case law on this varies and the subtleties on when the line is crossed - and still too - the degree of force applied is complicated. But a good defence lawyer, if you can afford one, should be able to play with this defence in court for you. I did a post grad paper that touched on this at law school (and although some years ago, the law is still relevant, if not more so) and am not just talking out my ****.
Although, as many have said, and particularly in federal countries such as the US, this can be different on a state-by-state basis.
My comment is that - and this is in no way any form of advice to anyone - if you genuinely feel you or a loved one are under imminent threat, act (reasonably/proportianately!! (that's the crux!)). Waiting to see what happens and waiting to see if you are effective and rapid enough to deal with a launched attack may be in some instances too late.