If you perceive a threat and the opponent has the ability and actions that inform your senses that the threat iof harm is going to happen then you have rights to self defense under the law. so pre-emptive strikes are allowed as long as you use the minimum amount of force needed to eliminate the threat.

if they are bigger than you then you can go a lot harder, if they have a weapon, if they say they have a weapon, if the threat is death etc.

you just have to stop before you become the bad guy/girl. as soon as they are unable to continue you need to stop.

sounds dumb, but if you know the laws related to attacks/defense/assualt etc it becomes easier to know when to fight and when to wait.
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everyones an expert on the internet. lol