Having discovered the location of Earth, the Covenant built up a massive invasion fleet of over five hundred ships to assault the human homeworld. This fleet was assembled by the Prophet of Truth in secrecy from the rest of the Hierarchs and the Covenant, and was commanded exclusively by the Jiralhanae instead of the Sangheili.
After discovering the plan from a piggyback transmission, Cortana and John-117 formulated a plan that would result in either an enormous strategic victory or the futile deaths of the remaining SPARTAN-IIs. The goal of FIRST STRIKE was simple: to attack the massive battle station Unyielding Hierophant supporting the Covenant fleet, and destroy it by sabotaging its reactors. While the operation was approved by Admiral Danforth Whitcomb, he was unwilling to risk either the Gettysburg-Ascendant Justice or the AI Cortana. The captured Covenant flagship was needed to warn Earth of the imminent invasion, while Cortana's memory was filled with vital intelligence recovered from Halo Installation 04. A plan was conceived to use a captured Spirit dropship and a disposable copy of Cortana instead.
During the operation at the suggestion of Lieutenant Elias Haverson, the Gettysburg-Ascendant Justice dropped out of Slipspace at the edge of the Tau Ceti system where the Covenant fleet had gathered. After observing how large the fleet was, Admiral Whitcomb had the hybrid ship relocated behind the moon nearest to the armada and devised an alternate plan. The Ascendant Justice's Slipspace drive moved to the Gettysburg, in preparation for a suicide mission. Using a captured Forerunner Crystal as bait, Whitcomb would draw as many ships as possible near the colossal space station Unyeilding Hierophant. The Ascendant Justice would then cause as much damage to the enemy as possible, luring them into the radius of the Hierophant's destruction.
In a desperate attempt to stop a massive Covenant armada heading for Earth, five Spartans infiltrated the Covenant battle station Unyielding Hierophant on board a captured Spirit dropship. On this mission, the Spartans would primarily be battling Brutes, who guarded a temple in an atrium-like structure in the battle station. One of the Spartan-IIs, Grace-093, was killed by a Brute Shot through COM triangulation tactics. John-117 then rigged Grace's suit to execute a fail-safe fusion reactor overload, killing all Covenant in a ten-meter radius (among which were a dozen Grunts and four Elites). The Spartans fought their way to a reactor room and set the generators to overload. With the station counting down to destruction, the Spartans returned to the atrium, opened the airlocks, and blew out a heavily enforced window exposing the station's interior to the vacuum of space. They then hijacked Banshees for use in their escape. This bold tactic resulted in the deaths of all nearby Covenant infantry. The infiltration of the Hierophant's data networks was orchestrated by Cortana's many back-up copies. Desperately evading counter-intrusion AIs, she nonetheless successfully opened the station's airlocks while tying up the Covenant's entire communications network.
The Spartans then traveled to the far side of a nearby moon and met the UNSC Gettysburg, now separated from the Ascendant Justice. Vice AdmiralDanforth Whitcomb and LieutenantElias Haverson unexpectedly took the Ascendant Justice and deliberately crashed into the Unyielding Hierophant. Using a hologram, Whitcomb revealed a highly prized Forerunner Crystal on-board his ship, and challenged all Covenant forces in the system to try and take it from him by force. The Covenant armada swarmed around the Ascendant Justice, desperate to take the Crystal from Whitcomb and ignorant of the sabotage of the Hierophant's reactors. The battle station exploded, destroying most of the Covenant fleet and saving Earth from an overwhelming attack. Before the operation, Whitcomb had spoken of the Alamo and the fate of its defenders: a tactical loss of every soldier present, but in the end, an enormous strategic victory.
The outcome and historical result of the battle was noteworthy, as the majority of the Covenant did not yet know the strategic importance of their intended target, Earth. Thus, their actual first entrance into the Sol System was with a much smaller force than the ill-fatedEpsilon Eridani System. This was crucial to the survival of the human homeworld and the significant UNSC presence on Earth, giving Lord Hood time to prepare for a bloody, and final siege. The survivors of the operation, including John-117, would later play a crucial role in the defense of the Sol system.
After using a spy drone to view what was left of the armada, the Gettysburg made a random jump away from the battlefield, and finally to Earth using the Ascendant Justice's Slipspace drive. There, the crew was able to warn Earth of the impending Covenant attack. Due to Whitcomb, Grace and Haverson's sacrifice, the Prophet of Regret attacked Earth with only a token force that was quickly defeated. Future Covenant attacks on Earth were met with the full force of the UNSC Homefleet, which included 300 Orbital Defense Platforms. Earth would not be taken by surprise, or without considerable effort on the Covenant's part.
0510 Hours: Master Chief officially presents the plan to the crew of the Gettysburg-Ascendant Justice. After much hesitation, Vice Admiral Whitcomb approves.
0530 Hours: The hybrid vessel passes through the Covenant armada in Slipspace. They are undetected, however a single Spirit launches from a hangar in the vessel. The dropship contains the SPARTAN-IIs: John-117, Fred-104, Linda-058, Grace-093, and Will-043. The dropship is undetected by the Covenant forces. The hybrid ship hides behind a nearby moon.
0610 Hours: The dropship is tugged and docked aboard the Unyielding Hierophant. The Spartans make their way through access pipes and conduits.
1751 Hours: The Spartan team enters a temple inside the station. They are ambushed by a group of Brutes. One of the first encounter of Brutes since the battle of Harvest in combat situation.
1755 Hours: Grace-093 is killed by a Brute Shot fired by the Brutes
1800 Hours: The Spartan team fights into the reactor room, where Cortana's copy initiates an overload of all 512 fusion reactors.
1810 Hours: The Gettysburg and Ascendant Justice unlink. The Ascendant Justice heads into the Covenant fleet, hoping to draw more Covenant vessels close to the station as it is detonating. Aboard are Vice Admiral Whitcomb and Lieutenant Haverson.
1820 Hours: The Spartan team uses Banshee fliers to escape the station.
1825 Hours: The Unyielding Hierophant explodes, destroying approximately 488 of the 500 vessels in the area and heavily damaging the remainder.
The Gettysburg was outfitted with the covenant slipspace equipment from the Ascendant Justice, this could possibly have made it the fastest UNSC ship at the time, due to Covenant slipspace technology being much more advanced than Human slipspace technology at the time.