If you are considering self-defense as a counter to assault charges, it is important to understand exactly what makes a physical altercation permissible as self-defense. One of the key components is a reasonable fear that someone else is going to do you harm.
In some cases, this is obvious. If someone strikes you during an argument, any fear you have of a physical assault is justified because it already happened. If you defend yourself, there is a good chance any charges against you will not stand.
However, this standard does mean that self-defense may be a valid response even if the other person did not harm you and perhaps never intended to. If you believe that they did intend to, you can still use self-defense.
For instance, perhaps the argument was with a man far larger than you, outside of a bar. You are both standing on the sidewalk, about five feet apart. You had no intention of taking things to a physical level, but then the other man threatened to beat you up and started walking toward you. From his demeanor and the verbal threats, you believed he meant every word. Instead of just letting it happen, you stepped forward and threw the first punch, knocking him out. You may still be able to claim it was self-defense even though you hit him first.
These cases can get complex, especially when there is no video evidence and both people offer different accounts of what happened. It is quite important for you to fully understand all of your legal defense options when you get to court.