Massacre of the Outer Colonies
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Harvest and humanity's response
CMA Argo was tasked with investigating the silence of Harvest, but signals with it were soon lost too. The Colonial Military Administration sent Battle Group 4 to investigate the escalating situation. Arriving on October 7, 2525, the Battle group was attacked. Only the CMA Heracles managed to escape. The Heracles returned to Reach on October 31st, relaying what had happened and a message: "Your destruction is the will of the gods, and we are their instrument."
The following day, on November 1, 2525, the Unified Earth Government began mobilizing the CMA and UNSC. Specifically, the UNSC went to full alert and stripped the CMA of its forces, giving them to NAVCOM and UNICOM. On November 2, the SPARTAN-IIs were deployed.
Early battles and losses
At some point immediately after the attack on Harvest, Green Hills and Second Base were attacked by the Covenant. On the 27th of November, Chi Ceti IV was assaulted by a Covenant ship at the same time SPARTAN-II forces were receiving their MJOLNIR Powered Assault Armor. Almost immediately after in December, the SPARTANs were reassigned to fight the Covenant at Alpha Corvi II.
The first year
The beginning of 2526 saw the destruction of Bliss. Soon after on March 1, Preston Cole in charge of UNSC Battle Group X-Ray fought the Covenant off over Harvest. Seven days later, Cole as well as others would devise Operation: SILENT STORM, part of which would involve laying a trap at Biko using Task Force Yama. At this point in time, the Covenant were only operating in the Polona Sector, Vevina sector, and Grenadi sectors. The colony of Etalan, with roughly 10 million people was also attacked during this time.
As part of SILENT STORM, the UNSC moved to position themselves at Seoba, over Biko and discovered an Insurrectionist plot to overthrow the Bikon Chancellor. This would require the UNSC to go on the offensive before they would fight the Covenant. The Insurrectionists were driven back, but not before the Covenant arrived and attacked the planet. Almost a month after, the prestigious Corbulo Academy of Military Science on Circinius IV was destroyed, leaving only 3 survivors.
Only two days after Circinius IV, ONI projects were refocused to solely counter the Covenant. As well, ONI Section Two sent emissaries to Outer Colony leaders to warn them of the Covenant and give them an offer: end hostilities with the UNSC and receive protection. Initially they laughed at the prospect, until they witnessed footage of Bliss.
Planets began falling two or three a week. The Office of Naval Intelligence was unable to collate intelligence at the rate of the Covenant incursion. UNSC Fleet Command was unable to plan out grand strategies and thus granted operational autonomy to UNSC naval units.
The Covenant kept pressing ever inward into human space. In 2528, the UNSC continued to counter the Covenant at the Battle of XI Boötis A but Madrigal fell later in the year. Two years later in 2530, Eridanus II was glassed and the UNSC abandoned the planet. On December 24 of 2530, the UNSC and Covenant clashed at the Battle of the Great Bear. The battle would lead to the formation of the Cole Protocol.
In February 2531, Arbiter Ripa 'Moramee, under the command of the Prophet of Regret, began leading searches for Forerunner technology to aid in the Covenant war effort during the end of the Harvest campaign and the Battle for Arcadia. With the Covenant having discovered the Forerunner shield world Etran Harborage sometime prior, the Prophet of Regret intended to use a fleet of Sojourner-class dreadnoughts docked at the Apex Site to crush humanity "quickly and with ease." With Ripa having kidnapped Professor Ellen Anders at Arcadia to use as a Reclaimer to activate the shield world, he was pursued by the UNSC Spirit of Fire, leading to a battle at the Etran Harborage. In the battle, Ripa was killed by Sergeant John Forge who subsequently sacrificed himself to destroy the Etran Harborage by causing its internal sun to go supernova. The Covenant plan was foiled, but the Spirit of Fire sacrificed its slipspace drive to cause the supernova, leaving the ship and its crew stranded in deep space.
The Inner Colonies
Continuing losses in the Outer Colonies & refugees