Recall also that "Yesterday's Enterprise" has already established the Klingon/Federation peace beginning 22 years prior to TNG with the sacrifice of the Enterprise-C. STVI ignored this. It also made McCoy and Uhura deeply incompetent, Kirk into a racist and Spock into a mind rapist. While also introducing a Klingon ability to fire while cloaked which they.... never bothered to try again.
IMO, STVI is a deeply flawed movie.
I have no doubt that Federation/Klingon peace wasn't driven by a single event. They're simply too different culturally, so peace was likely achieved in pieces over a long period of time.
How were McCoy and Uhura incompetent?
Having deep seated feelings of anger towards a group responsible for the brutal death of your son? Astonishing! Since the Klingons never renounced Kruge's actions in regards to Genesis, it would be easy for a person to be angry at a group based on those actions.
Spock the mind-rapist? Valeris was part of a cabal that was working to set off a war that could kill millions. The needs of the many...