In the late 2340s and early 2350s the Tholians had been testing the waters with their borders to the Federation, attacking some Federation outposts to see how the Federation would respond. The Federation sent a fleet, under the command of Admiral Taneko and his flagship, the USS Mizar, to stop them, leading to the Battle of Alphard in 2351.
The Admiral sacrificed his badly-damaged ship to destroy the Tholian fleet, winning the battle for the Federation. His legacy is his contributions to fleet tactics and organization which transformed the Starfleet strategic doctrine. (Decipher RPG module: Starfleet Operations Manual)