My post # 34 in this thread ends with this summary:
So there was a big and bloody war with the Klingons a few decades after "Errand of Mercy in TOS and sometime between 13 and 3 years before Star Trek VI: The Undiscovered Country. Either the Organians stopped keeping the peace during that interval, or else the Federation and the Klingons learned how to fight without going near enough to Organia to be noticed by the Organians.
Lord Garth, in post # 44 makes a case that the Federation and the Klingons were at war, though negotiating for peace, in
Star Trek III: The Search For Spock and in
Star trek IV: The Voyage Home. But it seems certain that the Federation and the Klingons were at peace in
Star Trek V: The Final Frontier since General Koord was able to order Captain Klaa
NOT to kill Captain Kirk. It would be highly unusual for a Klingon general to order a subordinate officer to not kill a Federation officer during a time of war, or for Starfleet and Klingon officers to mingle at a party during a state of war.
Therefore my theory is that the Federation and the Klingons were in a state of peace, but cold war and high tensions, during the movies from WOK to TFF, and that there were negotiations for a different peace treaty going on at the time.
But even so the evidence that there was a big war between the Federation and the Klingons before TUC indicates that the Organians kept the peace between the Federation and the Klingons for only a few decades. Either the Organians stopped keeping the peace during that interval, or else the Federation and the Klingons learned how to fight without going near enough to Organia to be noticed by the Organians.