Freeza's Force-Kanassan conflict
From Dragon Ball Encyclopedia
War for Kanassa
Main article: Dragon Ball Z: Bardock: The Father of Goku
For some time, Freeza's Force had been attempting to conquer Planet Kanassa. It was revealed that the planet held some form of secret energy that allowed its inhabitants to develop psychic abilities. Freeza apparently wanted to take hold of the planet in order to harness this secret energy. However, despite numerous attempts, even Freeza's elite squadrons were unsuccessful in conquering the planet. Somehow, the Saiyan Bardock and his team found their way to Planet Kanassa and joined in on the fight as a way to show Freeza that they deserved more recognition.
Bardock and his team, consisting of Tora, Fasha, Shugesh, and Borgos, waged battle against the Kanassans for a few days, waiting until the full moon finally arose to launch their final assault. The full moon caused the Saiyans to transform into Great Apes, which gave them the power to easily eradicate the population of Planet Kanassa within one night. After the battle had finished, only one Kanassan by the name of Toolo survived. He attacked Bardock and transfered his psychic abilities over to the Saiyan warrior, thus sparking a chain of events that would later lead to the Saiyan race's end. Bardock then slaughtered Toolo, thus finally killing off all of the population and allowing Freeza to take control over the planet.
A short time later, Zarbon, Freeza's right-hand man, informed Freeza that a band of low-class Saiyans had taken over Planet Kanassa in just a few days. Dodoria, another of Freeza's main henchmen, stated that the job to take Planet Kanassa had been in the pool for months, and that not even Freeza's elite squadrons were able to take Planet Kanassa so easily. This was the first time that Freeza was literally amazed at the growing power of the Saiyans. Freeza, Zarbon, and Dodoria contemplated the fate of the Saiyans due to the growing power of the race and decided to take action in order to avoid further repercussions, as the Saiyans might pose a great threat to them in the future.