What is the test for an assault on a peace officer?

The crime of aggravated assault on a peace officer requires that the officer be "engaged in the performance of his . . . duties" at the time of the assault. (Pen. Code, 245, subd. (d)(2).) The officer must be acting lawfully, because there is no "duty" for an officer to act unlawfully. Therefore, if the officer uses excessive force¹ or the detention is unjustified, a defendant may be found guilty only of the lesser offense of simple assault. (People v. White (1980) 101 Cal.App.3d 161, 166.) The "lawfulness of the arrest is an element . . . which . . . must be submitted to the jury. 'California cases hold that although the court, not the jury, usually decides whether police action was supported by legal cause, disputed facts bearing on the issue of legal cause must be submitted to the jury considering an engaged-in-duty element, since the lawfulness of the [officer] victim's conduct forms a part of the corpus delicti of the offense. [Citation.]' " (People v. Wilkins (1993) 14 Cal.App.4th 761, 778.)