A criminal complaint for assault was filed against me, so what are the requirements for self-defense to be established?

As in your situation, an act of immediately reacting to the other party's threat in order to defend oneself may be recognized as self-defense.

In the case of assault, an act to defend the legally protected interests of oneself or another from a present and unlawful infringement does not constitute an unlawful act when there is a reasonable ground, and it is therefore not punished.

However, where the defensive act exceeds the appropriate degree, the sentence may be mitigated or remitted depending on the circumstances.

The requirements for self-defense in an assault case are as follows.

① The legitimacy of the motive or purpose of the act
② The appropriateness of the means or method of the act
③ The balance of legal interests between the protected interest and the infringed interest
④ The urgency of the act
⑤ The subsidiarity, meaning that there was no other means or method besides the act

When these requirements are satisfied, the act may be recognized as self-defense and may not be punished as assault.

The police or the prosecution make a comprehensive judgment based on the specific circumstances at the time.

Therefore, a specific statement about the situation at the time of the assault and the securing of evidence are very important.

For the details, we recommend that you consult a criminal attorney and prepare a defense strategy for the assault charge.