An inconclusive battle between the Federation and the Klingon Empire near Donatu V, fought about 23 years before the tribble crisis of 2268.
The Battle of Donatu V was an armed clash between Starfleet and the Klingon Empire near the planet Donatu V, in the region around Sherman's Planet. It took place roughly 23 years before the events of The Trouble with Tribbles, which places it in the mid-2240s, during the long period of hostility that preceded open war.
The battle is known only from a brief reference in that episode, where the Federation and the Klingons are still disputing control of Sherman's Planet under the terms of the Organian Peace Treaty. No details of the fighting were given beyond the fact that it was inconclusive, which is why the area remained contested two decades later.