Why am I being charged with assault if they struck first and I was just defending myself?

You can be charged with almost any crime. In this case, it sounds like the man you got into a fight with went to the police and told them a different story than the one you have told me. Assuming there were no serious injuries, the charges are likely a misdemeanor assault. You will be given an opportunity to negotiate with the prosecutor and either reduce the charges to a plea deal you are willing to accept, or to take the case to trial. I have seen many cases like yours, where two individuals engaged in what we call 'mutual combat', and in general both parties are found not guilty of assault at trial, since it's impossible to prove who started it. You should hire a defense attorney to represent you in this matter, it sounds like a case you could win at trial.

Almost certainly there is a dispute as to the facts. Either the victim or a witness is claiming that you struck first. the prosecutor will let the judge decide what the facts are based on the testimony presented at trial.