The zerg Overmind was created by the xel'naga, an ancient race which was also a progenitor of the protoss. The zerg virtually destroyed the xel'naga, and when the Overmind absorbed many of them, it learned of the protoss, a race of powerful psychics with purity of form. It took the Zerg Swarm across the galaxy, intending to assimilate the protoss. However, it feared their psionic power, and intended to assimilate a species with such potential beforehand. It discovered such a species in the Koprulu Sector, in the shadow of protoss space–the terrans.
Course of the Battle
Alpha Squadron established itself in a base, west of the Sons of Korhal encampment. Shortly thereafter, zerg forces attacked near the Alpha Squadron camp, prompting Magistrate Collins to give a direct order to the commanding Alpha Squadron Lieutenant to destroy the xenomorphs.
The secretive Cerberus Recon Squad appeared on the battlefield, having been sent there to oversee the extermination of the xenomorphs...something Collins had not been previously informed about. Alpha Squadron and Cerberus worked together to defeat the rebels.
The defeated Sons of Korhal left the planet.
Battle of Flannum Installation
Magistrate Collins confirmed to Alpha Squadron that he had known about these xenomorphs for quite some time. They were being studied in the Flannum Installation but contact to the facility had been lost. Now the installation was surrounded by xenomorphs, which had created large, semi-organic structures, some of which were capable of defending themselves. Cerberus Recon Squad and Alpha Squadron cooperated in clearing a path to the installation.
The Cerberus Commander had access to the installation's codes (again surprising Collins), and used them to confirm the presence of surviving scientists inside.
A pair of Cerberus firebats entered the installation, intending to rescue the scientists. Instead, they ran into repeated ambushes by the zerg, some of which had been hiding under the ground and within the walls, but were able to band with the security guards within. Upon encountering the scientists, they ran into a trap, facing numerous zerg forces. However, the terrans were able to fight their way out of the installation.
Battle of Los Andares
Los Andares Prison, guarded by Los Andares Security forces, fell under attack. A team was dispatched to rescue the guards, although at least one prisoner took advantage of the situation to escape, leaving behind guard Lars Trakken. Three days later, the Confederate military rescued him, but unable to identity him, treated him as a mental patient.
The Confederate forces eventually defeated the zerg, beating them back from the capital.
Following their victory, Magistrate Collins and his advisers proposed establishing a tight defensive perimeter. However, the Cerberus Commander insisted on attacking the primary hive clusters, believing the time was right. When Collins disagreed, the Cerberus Commander imposed the Colonial Crisis Act under sub-section 1602, taking command of the whole operation.
The Confederacy defeated the xenomorph hive clusters.
Following the Confederacy's apparent victory of the aliens, the Cerberus Commander imposed a Class 7 Seal on all information surrounding the xenomorphs, threatening "immediate termination" for those who did not comply. The Confederacy controlled all communication and travel to the planet, hiding the events of the battles from the rest of their citizens.
Despite the Swarm's initial losses, the zerg soon overran Chau Sara's defenses, killing or infesting the majority of the colonists.
However, by the next month, Tassadar's Koprulu Expeditionary Force arrived in the system and discovered the infestation. The Conclave ordered the fleet to begin a purification operation against the planet. Though reluctant to carry out these orders, Tassadar obeyed. The Overmind ordered its forces to hold back so it could observe the operation.
The protoss began their attack by destroying all non-protoss orbital presences.
All life on the surface of the planet was extinguished.
The protoss specifically targeted population centers, hive clusters and secret installations.
Confederate colonel Edmund Duke launched a clumsy counterattack against the protoss fleet which was now heading toward the next infested planet, Mar Sara. Tassadar retreated out of respect for the terrans bravery, and resolved to destroy the zerg without indiscriminately killing the terrans as well.
The Confederacy flew into a state of panic over the attacks. In order to keep Mar Sara calm, a new magistrate was appointed for Mar Sara.
The Confederacy continued to limit information about the events at Chau Sara, though they did not hide the existence of the protoss.