If some drunk is in my face being abusive to me or my wife, I am not going to risk injury by waiting for them to throw the 1st punch. Mishael is spot on - you have a guard or fence, and if it is touched or broken, then I call that justification for a pre-emptive strike.
This is almost going down the 'human rights' argument. If people are going to throw their weight around and start picking on others for no reason, they lose that right. It is because no one stands up to them that they continue doing what they do until some innocent gets really hurt.