"Mr. Liberty, let me make myself perfectly clear. We have made first contact with another alien civilization. This contact consisted of them vaporizing the colony of Chau Sara. They burned it to the ground, and then burned the ground beneath it."
When the protoss discovered the extent of the zerg infestation of Chau Sara, ExecutorTassadar reluctantly ordered his expedition's ships to power up their weapons, destroying the planet, firing at both terran settlements and other points. At least two ships, firing massive energy beams, obliterated most or all life. A small number of marines, including Andre Madrid, survived the attack by hiding underground. Shortly after Chau Sara, the protoss fleet purified the planet of Gystt.
Tassadar avoided attacking the next infested world, Mar Sara, until after the terrans had evacuated. Several hours after the escape, Tassadar's forces again sterilized a planet.
Tassadar's fleet arrived at the Confederate capital world of Tarsonis, which had been besieged by zerg due to the Sons of Korhal's use of psi emitters. Lieutenant Sarah Kerrigan, a ghost, believed that the protoss arrived with the intention of destroying the planet. Forces under her command defeated the protoss. The protoss did not purify Tarsonis. Protoss and zerg continued to fight on the planet's core continent.
During or before the Second Great War, the zerg established a presence on Taurus IV, infesting the Dominion force sent to investigate. A protoss executor intercepted the distress call the terrans had sent out, and requested the aid of the Golden Armada. Protoss on the surface held the zerg at bay long enough for the Armada to arrive, which eradicated the zerg forces from orbit.
During the conflict proper, Doctor Ariel Hanson intended to relocate several hundred colonists to a world called Haven, a world ideal in all aspects aside from that it was very close to protoss space. After supporting Jim Raynor's campaign for a few weeks, she returned to Haven to find that the zerg had infested the world and several protoss warships had entered orbit to purify the planet. The protoss, recognizing the Hyperion, asked for Raynor to help in the purification, while Hanson asked for time to complete a cure for the infestation.
By 2511, the Terran Dominion began constructing bunkers that were buried beneath the reach of protoss incineration. OverqueenZagara set about regrowing worlds previously destroyed by protoss purification using the adostra.
According to Co-op Missions, fleets of the Protoss Empire were known for using great fleets to purify planets. However, the protoss largely took a policy of non-intervention, the Dae'Uhl prior to the Great War. Co-op also includes corsairs among these fleets, which as they were Nerazim were not part of the empire's fleets.