While reading about the POA I was astonished that it was state law to recite it - at least in Florida

. I always thought it would be voluntarily with most students saying it out of tradition, conviction and/or peer pressure.
While I want to leave the constitutional discussion to the Americans, reading the oath of allegiance raises a new question:
Why does it explicitly mention a prince? Wouldn't he already be covered by potentate or sovereignty? Or did the US had some beef with a prince at the time the oath was introduced?