CIS and Prequel era Jedi Order vs. Army of the Republic.
So... Dooku convinces Kenobi to switch sides at Geonossis. Kenobi convinces the Jedi to switch sides as well. They relocate (with all resources) to some CIS planet, and wage war on the Republic, who just recieved the clone army.
Sidious does not factor into the equation in this matchup.