The Tholian Incident was a fictitious event orchestrated by the Klingon Empire.
It was an elaborate scheme to allow a Klingon fleet to assault Earth and involved tricking James T. Kirk that he had arrived a hundred years in the future where the Great Peace had been achieved by the New Klingons.
A false history file stated that the 'incident' involved a Klingon and Starfleet force that bordered on their territories near Tholia. Though it appeared as though war was going to rage between the two sides, the Klingon fleet was actually manned by New Klingons that had travelled back in time in order to create a self fulfilling future peace. This would be achieved by a time displaced James T. Kirk along with the Organian Ayelborne. (TOS novel: Timetrap)