Was the self-defense claim doomed in the  Clifford case when he failed to retreat  to avoid the conflict with Vander Lee? The  best defense  for Sgt. Clifford  would have been  to simply walk  away and put an end to  the conflict.  

Minnesota law  authorizes the use  of a reasonable amount of force to resist  “an offense against the person.” See https://www.revisor.mn.gov/statutes/?id=609.06. The defense  has the burden at trial of coming forward with sufficient evidence to make self-defense an issue in the case. Once met, the burden  shifts to the prosecution to negate one of the following four elements of any self-defense claim: (1) an absence of aggression or provocation; (2) an actual and honest belief that imminent death or great bodily harm would result; (3) a reasonable basis existed for this belief; and (4) an absence of reasonable means to retreat or otherwise avoid the physical conflict.
A person can only assert a self-defense in an assault case   when  there is no alternative to the use of force. In other words, if Clifford had an opportunity to retreat from the confrontation  and failed to do so, the punch landed to the head of Vander Lee is not  considered self-defense. Did Clifford have the opportunity to retreat? It would seem so. The confrontation occurred in a public place with plenty of room to retreat. The victim was obviously intoxicated and   posed no real threat.  The defendant was a trained SWAT team leader with lightening quick  reflexes.  Sgt. Clifford could have easily taken one step backward and walked away the moment Vander Lee allegedly cocked his arm and readied his punch. The law always favors a peaceful resolution to conflicts over a  violent reaction.