The Battle of Donatu V, fought in 2244 in the Donatu system, was one of the few instances of open warfare between the Federation and the Klingon Empire. (TOS episode: "The Trouble with Tribbles"; TOS - Errand of Vengeance novel: Killing Blow) Although the Federation ultimately won the engagement, both sides suffered heavy casualties. The USS Yorkshire was credited with fighting the battle to a draw. (TOS novels: Constitution, Demands of Honor)