The First Battle of Geonosis, also referred to as the Battle of Geonosis or the Battle on Geonosis, was the first major battle fought in 22 BBY between the Confederacy of Independent Systems and the Galactic Republic on Geonosis, marking the beginning of the three-year Clone Wars. It would be the first major combat of the Grand Army of the Republic, as well as the first major battle the Jedi had fought in years. The battle also caused the death of the notorious bounty hunter Jango Fett and the discovery of Count Dooku‘s dark side allegiance.

A group of 212 Jedi, which was the largest deployment of Jedi since the High Republic Era, including Coleman Trebor, Eeth Koth, Kit Fisto, Plo Koon, Even Piell, Shaak Ti, Luminara Unduli, Aayla Secura, Barriss Offee, and Ki-Adi-Mundi were tasked with rescuing Obi-Wan Kenobi and investigating the reports of Separatist warmongering on Geonosis by the Jedi High Council. Despite this large number of experienced Jedi, among the group were many new Jedi Knights who lacked the experience of their masters. Among these newer knights was Bultar Swan, who later stated that «The battle wasn’t supposed to happen.» She also said that she knew it possibly could, but she expected the enemy to surrender as soon as the Jedi ignited their lightsabers.

The Zilkin Meebur Gascon participated in the battle, as part of Republic Strategic Command, by analyzing Geonosis’s hostile terrain and recommending an effective attack plan.

Skywalker, Amidala, and Kenobi managed to avoid the creatures that had been set upon them long enough for Master Windu’s Jedi strike team to show up. As the Jedi came into view throughout the arena, many Geonosians chose to flee rather than face them in combat. However, Count Dooku deployed the Separatists’ new battle droid army, beginning a violent battle took place in the arena as the droid legions were unleashed. The first batch of B2-series super battle droids were part of the army. Though outnumbered, the Jedi fought bravely even as their ranks were slowly diminished in the face of an unending number of battle droids. Jedi Stass Allie and Bultar Swan would fight back-to-back during the battle against the droid army.

It was during the fierce battle that Padawan Tan Yuster was overwhelmed and killed by super battle droids, and bounty hunter Jango Fett attempted to kill the famed Jedi Master, Mace Windu. However, during their confrontation, the Reek charged Fett and nearly crushed him; although Fett slayed the beast, its attack had broken his jetpack. When the duel resumed, Mace Windu charged straight towards Fett, deflecting his blaster shots. At the last second, the Mandalorian attempted to activate his jetpack, but it backfired, allowing Mace to quickly decapitate the notorious bounty hunter disturbing Dooku in the process. His damaged helmet was soon found by his «son» Boba Fett after the conclusion of the arena battle.

An unexpected participant in the battle was the 3PO-series protocol droid C-3PO, whose head had been detached and welded onto a battle droid’s body while a battle droid’s head had been attached to his body. Both incarnations walked into the battle alongside the battle droids and were knocked out by Kit Fisto and another Jedi. By the end of the battle, the R2-series astromech droid R2-D2 was able to locate C-3PO’s head and reattach it to his original body.

Despite the bravery of the Jedi, Dooku’s droids held the upper hand however, due to the legion of battle droids significantly outnumbering the Jedi. Many Jedi were slain by the droids’ blaster fire, and when the last of the surviving Jedi became surrounded, Dooku called a halt to the fighting. The Separatist Head of State offered the Jedi the chance to surrender, but Windu declined, saying the Jedi would not become hostages with which Dooku could barter. Dooku then apologetically ordered the surrounding droids to slaughter the Jedi, only for Yoda to arrive overhead with a regiment of clone troopers.

Launching a somewhat desperate assault into the arena in waves aboard LAAT/i gunships, the sudden arrival of the clone army saved the survivors and created a perimeter for long enough that they could board and escape. Yoda’s squad and the other troopers landed, and, during this arena battle, enormous Republic assault ships had penetrated the upper atmosphere of Geonosis, knocking out the Geonosian launch hives with aerial bombardments and clone commando strike teams and clearing the way to deploy the Grand Army of the Republic. The clones used their sudden, overwhelming force to their advantage instead of relying on precision in the arena. As Jedi General Yoda and the survivors evacuated the vacant arena cluttered with droid parts, the first battle of the Clone Wars began.

General Grievous was in the stalgasin hive around the time of the arena battle, though entirely by accident, and as he retreated through the hive tunnels beneath Geonosis, he encountered several Jedi, clone troopers, preventing them from reaching the Separatist Council. No one was left alive to speak of his existence until much later he emerged from hiding.