In its earliest ages of civilization, Onderon was a world marked with savage jungles that were filled with dangerous creatures. To survive, the primitive human inhabitants of the planet protected themselves behind the walls of fortified settlements. Over time, these grew to become enormous walled cities, like the planet's capital, Iziz. In the modern era, during the time of the Clone Wars, the planet's monarch King Sanjay Rash allied Onderon to the Confederacy of Independent Systems. Many Onderonians, however, refused to recognize the legitimacy of Sanjay's rule -- they preferred the neutral stance championed by former monarch Ramsis Dendup, whom Rash ousted.
King Ramsis Dendup refused to commit Onderon to the Clone Wars, thinking that neutrality would spare the planet from the violent conflict. Sanjay Rash knew better. To ensure Onderon's prosperity and strength in the shifting galaxy would require a strong alliance -- and that meant striking an accord with the Separatists. Through methods cloaked in intrigue, Rash assumed the crown of Onderon, deposing Dendup and branding him a powerless old crone. Rash then allowed the Separatists to establish a firm foothold, cementing his rule of the planet.
During the Clone Wars, the Galactic Republic had little recourse with worlds that willingly sided with the Separatist Alliance -- the rules of engagement enforced by the Senate and the Jedi Order contained the battlefield to contested worlds. But what of planets where the Separatist banner did not garner unanimous support among its populace? When an insurgency on Onderon refused to recognize the legitimacy of its monarch and began a rebellion to oust both the king and the Separatist influence on the planet, Anakin Skywalker saw an opportunity: If the Jedi could train and equip the rebels on Onderon, these freedom-fighters would keep the Separatists occupied during the Clone Wars.