I was wondering what constitutes a provocation to which it is justifiable to use force as self defense. For example, if someone came a slapped you on the face but did not proceed hitting you (at which point it could be claimed he no longer presented threatening force to you), would it be okay to punch him and not be legally liable? Or if someone started getting in your face but not really touch you, would it be legally okay to punch them to get them out of your face? What are some examples of provocation that the law would be sympathetic with the target using physical force against?
In general, what are some guidelines, in the U.S., to know when it is okay to respond with physical force? Is ANY violence (e.g. a humiliating act like slapping somebody) enough to exempt the victim from ... beating the c**p out of or shooting the perpetrator? At which point does shooting the perpetrator becomes fair game because their actions constituted a legitimate threat?