I would like to add something to this...
There are times when leaving is not an option. Something precludes you from escape. At times like that, when you can
articulate why you felt you or another were in imminent physical danger. They key is being able to justify a pre emptive strike with "resonable necessity". You must be able to show why what you did was necessary, and how it was resonable. This means that for self defense, you get away as soon as possible, not stick around to give the guy a couple of well deserved kicks to the ribs.
Also, a real key is to be able to express why you were afraid. This goes with the "resonable necessity" too...what did the other guy do that made you afraid. It doesn't sound macho to admit fear, but it does give a strong legal defense if you start out by saying, "I was in fear for my safety, and this is why."
http://www.revisor.leg.state.mn.us/stats/609/06.html This link goes to the statute for the state of MN regarding use of force. I would strongly encourage you to check your state statutes to find out if they allow for a pre emptive strike or not.