Darth Tyrannus led the
Confederacy at the
Battle of
Coruscant during the
Clone Wars. However, the fallen
Jedi, well-known for his
intelligence,
class,
wealth, and sheer
power, would suffer an embarrassing and crushing
defeat at the hands of
General Obi-Wan Kenobi's
Grand Army of the
Republic.
Kenobi's daring plan to rescue the Supreme Chancellor Palpatine was a success; however, Kenobi was unaware that Palpatine was pulling the strings behind his own kidnapping and that he was behind the machinations of the Grand Army as well as the Confederate Army. The Battle of Coruscant was really a trap designed by the Chancellor to see if the Chosen One Anakin Skywalker could be turned to the Dark Side. Count Dooku was privy to his master's plans, with the exception that he realized only too late that Palpatine was permanently replacing him. Palpatine had promised Dooku that if Anakin gained the upper hand, he would join the fight. He was lying however. He was just using Dooku, and Dooku learned only too late the Sith's nature of turning against each other. Dooku, also known as Darth Tyrannus, was a great warrior but he had the supremely bad luck of going against the Chosen One, Anakin Skywalker, potentially the most powerful Jedi or Sith of all time, just as Anakin was hitting his prime.
Tyranus himself was executed in one on one combat by the legendary Jedi Knight, who would eventually succeed him as Dark Lord of the Sith. Tyranus was also succeeded by General Grievous in his position as leader of the Confederacy. However, the Confederacy was never able to recover from Dooku's death, and the Republic won the Clone Wars. The "victory" was a pyrrhic one though because the head of the Republic, Palpatine, was also a Sith Lord. The Republic was quickly reorganized as the Galactic Empire after the War.
In fairness to Dooku, he should probably be considered one of the top 10 Jedi as well as top 10 Sith Lords of all-time. He completely demolished Kenobi in personal combat in both his confrontations with him. He also went toe to toe with his old master Yoda and held his ground before deciding to run away. Had he been a little more ambitious he might have recruited Anakin himself and overthrown Darth Sidious/Chancellor Palpatine, thus allowing himself the freedom to implement his own visions for the Jedi Council and Galactic Republic, two institutions which he obviously was gravely intent on reforming. Why else would he have conspired with Palpatine to start the Clone Wars? So close but no cigar. His perverted idealism was ultimately betrayed by the selfish ambition of Palpatine.
Dooku converted to the Dark Side soon after the Death of Darth Maul at the Battle of Naboo. He may have been disillusioned by the death of his apprentice Qui-Gon Jinn (who in turn was Kenobi's master), a death which he apparently blamed on the Jedi Council.