Operation: FIRST STRIKE was a mission undertaken by John-117 and several other Spartan-IIs to infiltrate and destroy the Covenant space station Unyielding Hierophant and associated armada of over 500 warships in the Tau Ceti system prior to the Covenant's invasion of Earth.
Following the events on Installation 04, John and the remaining survivors commandeered the Covenant ship Ascendant Justice and returned to Reach to search for survivors from the attack a few weeks earlier, including several Spartan-IIs that had fallen out of contact during the initial battle. In the process, Cortana learned of a massive Covenant fleet amassing for a planned invasion of Earth. After amassing what few people they could, The Spartans devised a plan to use a Spirit dropship to drop out of slipspace near the Covenant armada while the rest of the crew planned to return to Earth.
While infiltrating the station, the remaining crew returning to Earth changed their plan and joined the Spartans in the area to assist in the attack. Once the Spartans set the station's reactors to overload, Vice Admiral Danforth Whitcomb used part of the return vessel to lure most of the Covenant armada towards the Unyielding Hierophant as it overloaded and destroyed most of the 500-ship fleet. The actions taken by John-117 and the Spartans, as well as Vice Admiral Whitcomb significantly hampered the Covenant's invasion of Earth, and likely led to the UNSC's victory at Earth over the Covenant.
These events are depicted in the novel Halo: First Strike.