Around 9 BBY, the Outer Rim world of Geonosis was sterilized by the Galactic Empire, killing most of the planet's population, by orders from Grand Moff Wilhuff Tarkin, to ensure the secrecy of the Death Star project. The Geonosians of the Stalgasin hive were the original builders of the Death Star, an armored battle station capable of destroying entire planets. During the Clone Wars, they were persuaded to aid in the battle station's construction by Lieutenant Commander Orson Callan Krennic.
At the end of the war, Supreme Chancellor Palpatine declared himself Emperor and transformed the Galactic Republic into the Galactic Empire. The Death Star project fell under Imperial control and the battle station was moved away from Geonosis. The only survivor of the sterilization was Klik-Klak, who at some point after came into the possession of a Geonosian egg who when hatched became Queen Karina. She eventually moved to a lair beneath the planet's surface. Unable to produce new offspring and thus unable to repopulate the species, Karina continued the production of battle droids that the Geonosian droid factories once produced. The Sith Lord Darth Vader came to Geonosis during the Galactic Civil War and took the droids from her.
The Death Star was designed by the Geonosians.
Prior to the outbreak of the Clone Wars, the Geonosians designed an Ultimate Weapon for the Confederacy of Independent Systems. This weapon, later known as the Death Star, was an armored battle station capable of destroying entire planets. When the Clone Wars broke out on their homeworld of Geonosis, the designs for the Ultimate Weapon were taken by Count Dooku, a Sith Lord and leader of the Separatist Alliance, to his master, Darth Sidious, on Coruscant. Sidious was secretly Supreme Chancellor Palpatine, who led the Galactic Republic and controlled both sides of the war. His goal was to transform the Republic into a Galactic Empire and rule the galaxy as Emperor.
Sidious' goals came to pass at the end of the Clone Wars. The Separatists were defeated, and the Republic was transformed into the Empire. The Death Star continued its construction over Geonosis under the supervision of first Vice Admiral Dodd Rancit, and later Moff Wilhuff Tarkin. After whispers of the Death Star and its location over Geonosis managed to get out into the wider galaxy, Tarkin, now a Grand Moff, ordered the station move to a new location in the name of keeping it secret. Ultimately, the station was brought to the isolated world of Scarif. After the Death Star reached Scarif, Tarkin ordered the Imperial Weapons division to sterilize Geonosis in the name of silencing any potential loose ends who could reveal more about the super weapon. On Tarkin's order, the Imperials used Imperial gas canisters to spread toxic insecticide. The attack destroyed all but one of the planet's population of approximately 100 billion, resulting in the largest genocide ever committed by the Empire.
The rebels find the remains of the Death Star's construction and learn that the Geonosians were wiped out.
Within months of the genocide, the entire Death Star project was up and running over Scarif. In 3 BBY, the third year before the Battle of Yavin, the rebel crew of the starship Ghost traveled to Geonosis after receiving an intelligence report suggesting that the Empire was building something over Geonosis.
When they arrived, they found only Imperial construction modules that had once supported the Death Star's construction, though the rebels were unaware of what was being constructed there. The intelligence was, in reality, a trap to lure the rebels to Geonosis. Agent Kallus of the Imperial Security Bureau led a group of stormtroopers in ambush against the rebels on one of the modules, but the rebels were able to escape. One of the rebels, Garazeb Orrelios, was stranded on Bahryn, an icy moon of Geonosis, with Agent Kallus after an escape pod they were in crashed. Before they were rescued, Orrelios learned that Kallus was unaware of what happened on Geonosis. He did not believe that the Empire had any reason to exterminate an entire population, but Orrelios said he was simply in denial.
The following year, Senator Bail Organa dispatched Saw Gerrera and his Partisans to investigate the apparent disappearance of the Geonosian species. Saw and his team were ambushed and attacked by Klik-Klak, who mistook them for hostiles. After Rebel Command lost contact with Saw and his team, Senator Organa dispatched the Spectres and the former Clone Captain Rex to continue Saw's mission. After reuniting with Saw, the Spectres and Rex captured Klik-Klak. A disagreement ensued between Saw, who wanted to take Klik-Klak offworld for interrogation, and the Spectres, who wanted him to remain on Geonosis.
The rebels were forced to take their ship Ghost into the depths of Geonosis after they were attacked by a light cruiser commanded by Captain Brunson. Down there, the rebels discovered the poison canisters which the Empire had used to perpetrate genocide against the Geonosians. The rebels managed to flee Captain Brunson's cruiser but were unable to bring back physical evidence of the poison canisters. However, the rebellion was able to use visual images of the canisters to rally more worlds to the rebel cause.
Aphra looks over the remains of Geonosians.
Several years later, during the Galactic Civil War, the Sith Lord Darth Vader and the archaeologist Doctor Aphra went on a secret mission to Geonosis in order to acquire battle droids for use in his personal army. They found Karina and discovered that she was continuing to manufacture battle droids, as the Geonosian droid factories once did before and during the Clone Wars. Vader took her battle droids, but left the queen alive. The rebel leader Mon Mothma's Declaration of Rebellion indirectly referenced the Geonosian genocide when it condemned the Galactic Empire for instituting a "blatant policy" of speciesism and genocide against the non-human populations of the galaxy.
It was implied on Rebels Recon that the genocide was committed to keep the construction of the Death Star over Geonosis a secret. Such was confirmed by Dawn of Rebellion.
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