Master Yoda's greatest defeat was a secret one, long before the events of Star Wars: Episode III - Revenge of the Sith. Famously considered one of the wisest Jedi, by the time of the prequel trilogy Yoda had become Grand Master of the Order - the one whose voice was most respected, whose counsel was sought by the other Masters and whose wishes were seldom overruled. As great as Yoda may be, though, Star Wars has made it clear Yoda was a failure; the Sith returned on his watch, after all. And yet, for all that's the case, his greatest - and most personal - defeat happened years before Star Wars: Episode III - Revenge of the Sith.
Cavan Scott's Dooku: Jedi Lost revealed Count Dooku's backstory, transforming him into a tragic figure who was drawn back into the affairs of his homeworld Serenno even as he grew disillusioned with the Jedi Council. Star Wars has never really explored the emotional impact of this on Master Yoda, the pain the Jedi Grand Master would have felt as the Apprentice he shared a sacred bond with left the Order. Even more disturbingly, Claudia Gray's Master & Apprentice hinted Dooku began exploring the dark side shortly after leaving the Jedi; if Yoda sensed this, he would undoubtedly have been heartbroken.
There's some evidence Palpatine began to train Dooku long before Maul revealed himself to the Jedi in Star Wars: Episode I - The Phantom Menace; one scene in Claudia Gray's novel Master & Apprentice, set seven years before Qui-Gon Jinn discovered Anakin Skywalker on the sands of Tatooine, saw Dooku attempt to tempt other Jedi to him in researching the dark side. Palpatine cared little for the Rule of Two, viewing his Apprentices as pawns, and no doubt he felt recruiting Dooku was a secret victory over Yoda. While he was surely drawn to Dooku for his political connections and personal charisma, Darth Sidious would have been delighted at the idea of corrupting Yoda's old Padawan.
The relationship between Yoda and Dooku hasn't really been developed in Star Wars canon, but in the old Expanded Universe Sean Stewart penned the excellent novel Yoda: Dark Rendezvous. This saw Dooku coax his old master away from the Jedi Temple on Coruscant by proposing secret peace talks, and Yoda's love for his former Padawan combined with his eagerness to bring the Clone Wars to an end. It's no longer considered canon, though, which means it would be possible for new writers to tackle the dynamic. Hopefully Star Wars will indeed build on this theme, showing Yoda's relationship with his Padawans - and shining a light on his greatest, most personal, defeat.
