The Rittenhouse Scandal refers to an attempted coup d'état by Starfleet Vice Admiral Vaughan Rittenhouse in the year 2270.
Admiral Rittenhouse saw the many civilizations in the galaxy that were hostile to the Federation (specifically the Klingon Empire) as threats too great to be placated or ignored any longer, so he planned to subjugate those forces by using a militarized Starfleet with the dreadnought USS Star Empire as a spearhead. To be able to carry out his plan with no interference from civilian authorities, Rittenhouse plotted to overthrow the Federation government by placing officers loyal to him in key positions within Starfleet and the Federation Council.
Before his plan could be fully realized the Star Empire was stolen by a group of officers determined to keep Rittenhouse from using Starfleet to impose order by force. Rittenhouse later did battle with the Star Empire and the USS Enterprise but was defeated. His plot exposed, Rittenhouse committed suicide by destroying his ship, the USS Pompeii. (TOS - Fortunes of War novel: Dreadnought!)
Rittenhouse's actions were very damaging to the reputation of Starfleet. Admiral Heihachiro Nogura was tasked with repairing the damage done to the reputation of the fleet. (TOS - The Lost Years novel: The Lost Years)