Hypothetical scenario: A victim is identified with documented injuries consistent with having been struck with a weapon. The police find and charge a person (now "Defendant") that they claim perpetrated the act of striking the victim with a weapon. This Defendant stands trial for the crime of "Assault."
Common Law provides a number of Criminal Defenses that could be raised in this scenario (e.g., Self Defense, Duress). However, in order to raise such a defense the Defendant would have to effectively confess to the Assault (or at least some critical elements of it). E.g., a Defendant can't argue, "I feared for my life" without, in effect, admitting that he was present and capable of striking the victim.
Can the Defense require that the fact-finder in a trial (typically a jury) rule on prefatory facts, before it finishes the defense?
In this hypothetical, it seems prejudicial (and something like a violation of a Defendant's fifth-amendment right to not witness against himself) to require that the Defendant admit that he was present at the scene of the crime in order to invoke a defense.
It seems more consistent with the principles of our legal system to allow the Defense to require the Prosecution to first prove beyond a reasonable doubt that the Defendant was at the scene of the crime. Because if the Prosecution can't meet that hurdle, then that's the end of the trial.
In practice is it possible for the Defense to avoid presenting affirmative defenses (like Self Defense) before the Court has found beyond a reasonable doubt that the Defendant committed the crime? If so: How?
If not: Is there some theory or principle that illuminates why this is not considered a violation of the Defendant's fifth-amendment rights?