The battle above Khitomer in 2293 in which the Enterprise-A and the Excelsior destroyed General Chang's prototype Bird-of-Prey, able to fire while cloaked, and so saved the peace conference.
The conspirators who killed Chancellor Gorkon planned a second assassination at the Khitomer conference, this time of the Federation president. Chang's ship waited in orbit to stop the Enterprise from interfering.
In Star Trek VI: The Undiscovered Country the invisible vessel pounds the Enterprise at will while Chang recites Shakespeare. Captain Sulu brings the Excelsior to its aid and is struck too. Spock and McCoy then adapt a torpedo to home in on the plasma exhaust of the cloaked ship, using equipment carried for charting gaseous anomalies.
The torpedo finds its mark, and both starships finish the Bird-of-Prey. Kirk's party beams down in time to stop the assassin, and the plot by officers of both powers is revealed to the delegates.