Invasion of Algolis

The Invasion of Algolis, also known as the Battle of Algolis, was a prolonged campaign on the planet of Algolis from October to December of 2551. Primarily fought on and around Algolis, the entire system was embroiled in conflict until the Covenant's eventual retreat on the 29th of December.
==Background==
During a previous engagement, the Fleet of Cleansing Declarations had managed to plant a tracking device on the Hillsborough-class destroyer Alfa Romeo which had arrived in the Algol System in September to undergo minor repairs. Discovering the existence of a human colony on Algolis, the Covenant immediately arrived in the system and began to deploy forces to engage the planet's defense fleet.
In addition to the already extant garrison on the planet, a large number of Marines were rapidly redeployed to Algolis from nearby systems to reinforce it as news of the planet's attack reached UNSC HIGHCOM. 
==Engagement==
===Urban Asasults===
The initial wave of attacks on the surface of Algolis included a number of assaults against the planet's urban centers by Covenant airborne forces. Platoons of Covenant were deployed in waves and quickly began to overwhelm the defenders of many of the urban centers of Algolis. 
===Battle of Weapons Research Facility T12A===
The Covenant had discovered the existence of WRF T12A shortly after the commencement of their invasion of the planet and committed a sizable force to assault and capture the highly valuable facility. Several Marine platoons formed a perimeter around the facility's outskirts to allow for the evacuation of personnel and to give Hades Corps time to destroy a valuable asset to prevent it's capture.
The situation quickly began to deteriorate; the appearance of a massive force of Banshees caused heavy casualties among the defenders and atleast two cargo transports carrying civilian and military personnel were shot down. Rapidly-arriving Covenant strike teams aboard Phantoms gradually pushed back the collapsing perimeter defenses with the assistance of several armoured vehicles. In order to buy time for his corps to evacuate, 'Ghost' commandeered the Mark I ADS and engaged the enroaching Covenant forces, dealing heavy casualties to them and holding them back long enough for the remaining evacuation transports to escape the facility. He inevitably began to falter, the ADS taking heavy damage and being rendered immobile, though once he had noticed that the transports had successfully escaped he accepted his fate and activated the suit's self destruct sequence, the resultant blast obliterating all Covenant forces in the surrounding area and wiping the facility out.
===Heart of Midlothian===
Shortly following the engagement at Weapons Research Facility T12A, the Halberd-class destroyer Heart of Midlothian, which had just arrived from slipspace, was boarded by Covenant Special Operations personnel. After a short firefight throughout the ship, the Covenant seized control. The vessel's AI Mo Ye found itself unable to destroy the ship and enact the Cole Protocol due to the survival of the ODST Michael Baird. After being killed by an elite, Mo Ye detonated the vessel which destroyed the boarding forces and a nearby CCS-class battlecruiser along with it's complement.
===Alfa Romeo vs. Rapturous Transcendence===
Concurrently to the battle aboard the Heart of Midlothian, the CCS-class battlecruiser Rapturous Transcendence, which was harassing the ongoing withdrawal of civilians, was attacked by the Hillsborough-class destroyer Alfa Romeo which had been assisting in the evacuation of one of the planet's urban centers from orbit. Both vessels began to engage eachother at long range, the battlecruiser's plasma torpedos delivering significant damage to the Alfa Romeo while the latter's weapons were unable to penetrate it's foes' shielding. After both sides briefly disengaged to reorganise and prepare for another attack, the Alfa Romeo began to employ a risky tactic; it intended to close the distance between it and it's enemy. 
The Alfa Romeo fired a MAC round which did extensive damage to the Rapturous Transcendence's shields but was insufficient to neutralise them. Regardless, it managed to close the gap between it and the almost-stationary battlecruiser and grazed it's portside, which very temporarily neutralised the already-damaged shielding of the cruiser. Next, now under heavy plasma fire, the Alfa Romeo deployed it's entire arsenal of Rudra-class nuclear missiles which were configured to travel at a speed identical to that of the Rapturous Transcendence. The UNSC vessel now disengaged, deactivating it's main engines and subsequently dropping behind the enemy battlecruiser, which was engulfed by the detonation of the missiles which was contained by the now-recharged shield. After the destruction of the Rapturous Transcendence, the heavily damaged Alfa Romeo limped away from the system, disengaging from the battle raging within.
===Further Ground Engagements===
Despite having soundly defeated the UNSC's fleet in orbit and having achieved several major victories against them on the ground, the Covenant began to see major setbacks across the board. In particular, a human counteroffensive had recaptured all but one of the planet's major cities which had previously been overrun. In response, the Covenant began to glass the planet's urban centers which forced the UNSC to withdraw to the rural regions of the planet.
====Mission to Gustav Ridge====
In order to supplement their ongoing efforts to root out the UNSC defenders of Algolis, the Covenant established several major refueling depots across the planet. The largest and therefore most important of which was located at Gustav Ridge and was causing significant trouble for the marine defenders of the region. The Office of Naval Intelligence ordered the clandestine deployment of Rose-788 in an effort to turn the tide of the battle, and devised a plan of action which included the destruction of the refueling depots and therefore the disruption of Covenant ground logistics.
Rose-788 alongside the marine GX Company were deployed to attack and destroy the Gustav Ridge depot, however the operation suffered a major setback as the CCS-class battlecruiser Indulgence of Faith arrived at Gustav Ridge to resupply the base. Despite being urged to withdraw by the mission's ONI overseer, Rose and GX Company pushed on through the mountainous region until they reached Gustav Ridge. GX Company's demolitions specialists detonated a section of the ridge overlooking the refueling depot, causing heavy damage to it shortly before the main assault began. With the Covenant already in disarray, the depot was cleared of hostile forces rather quickly and a number of explosives planted throughout the base; though a new issue presented itself. With the Indulgence of Faith still hanging over the depot, oblivious to the occurrences on ground but still maintaining a gravity well, it was possible that Covenant reinforcements could arrive at any time and wipe out GX Company. The base's landing zone had been severely damaged by the collapsed portion of the ridge, and the UNSC was unable to deploy a pelican due to the battlecruiser's presence, meaning that the team had two options; leave on foot and deal with Covenant air patrols returning to the base or board the Indulgence of Faith and acquire transport.
=====Battle Aboard the Indulgence of Faith=====
GX Company and Rose-788 ascended the Indulgence of Faith's gravity well, arriving in it's cargo bay which was notably empty; most of it's non-essential complement was engaged in ground combat across the planet. Their presence was quickly detected and a large number of small rapid-response combat teams were deployed to fend off the boarders, but were ultimately unsuccessful, not being able to prevent the Marines from reaching the hangar. Meanwhile, Rose-788 split off from the main group to locate the ordnance bay with the hope of acquiring an Antimatter charge. Concurrently, GX Company held the hangar against the crew's attempts to neutralise them, and even despite the depleted size of the ship's military complement they began to prove too numerous for them to handle. After locating and priming a number of Antimatter charges within the ship's ordnance bay, Rose arrived in the hangar where she assisted the rapidly-losing GX Company in holding off the Covenant until they could commandeer a phantom. Safely away from the ship, the Antimatter charges detonated and destroyed the Indulgence of Faith and most of Gustav Ridge.
====Depot Campaign====
With the most important Covenant supply depot now eradicated, the UNSC's attention turned towards the remaining depots on the planet. With the loss of the Indulgence of Faith, a major wrench had been thrown in the Covenant's plans to conquer Algolis, and though the fleet remained intact it had very few vessels it could spare to re-establish and maintain new supply depots.
Over a period of three days, coordinated assaults by the Marine Corps of Engineers saw the destruction of over half of the Covenant's planetary depots. Army units ALG01 and ALG02 launched an unsuccessful assault against one of the invaders remaining depots, but in the process they discovered that the Covenant had been slowly attempting to reinforce their remaining depots. With this new time constraint, the UNSC had to act fast if it was to successfully root out the remaining bases.
The marine companies GX and AL were assigned to assault a depot with the assistance of Rose-788. They were aerially inserted into the depot as UNSC aerial forces contested the skies above, giving them a short window of time to get in and get out before extraction would become impossible. After clearing the base of hostiles, specialists from both companies planted high-yield nuclear charges within the depot, but a surprise attack from Covenant aerial reinforcements wiped out AL company and put GX Company on the backfoot. Rose-788 and the remnants of GX were split, unable to reach eachother and forced to deal with the reinforcements on their own. Soon, GX Company was wiped out and the arrival of Covenant banshee squadrons saw the UNSC quickly lose the battle raging in the skies. Now alone and against an increasingly large number of Covenant reinforcements, the lone Spartan had to manually prime the charges for detonation. Rose boarded a phantom whilst it was deploying it's troop complement and escaped, with the depot being eviscerated by the nuclear detonation over an hour later.
===Covenant Withdrawal===
Both sides had suffered greatly by the later stages of the Invasion of Algolis, but despite their heavy losses the UNSC still retained uncontested control over the planet's surface. Whenever the Covenant would establish a new forward operating base or supply depot, Marine Specialists would raid and destroy it. When the Covenant attempted to attack the UNSC's now-dug-in positions, they would be decisively beaten. This made the battle untenable, and lacking the ability to glass the entire planet without becoming vulnerable to ground-based countermeasures, the Covenant opted to withdraw from the system. All Covenant ground forces converged on their last remaining planetary base at Andramane Field, which caught the attention of UNSC reconnaissance teams.
While the evacuations were ongoing, a massive combined-arms assault on the Andramane Field base was conducted by the Marine Corps and the Army. Armoured divisions quickly secured the base's perimeter and aerially-inserted units overran the base within hours, the attack coming as a complete surprise to the Covenant defenders who were immediately flung into disarray. Survivors of the battle were evacuated via phantom to the fleet in orbit, which withdrew into slipspace shortly after. While the attack on Andramane Field was a decisive victory, the casualties sustained and the strategic value of the battle made it's necessity questionable and it's losses preventable.
==Aftermath==
In the immediate aftermath of the Covenant's withdrawal from Algolis, the UNSC was left with a highly war-torn planet that was undoubtedly going to see a second invasion attempt by Covenant forces. As the war continued to advance deeper into the Inner Colonies, Algolis became less and less strategically important by the day, and it was decided in January of 2552 that the planet must be abandoned. 
The massive casualties sustained by the UNSC and it's abandonment of the planet a mere month after their victory made the ordeal very controversial. Even though they had successfully fended off a Covenant invasion which was rather uncommon at this stage in the Human-Covenant War, they had achieved practically nothing except the loss of an extremely high amount of men. Many of the survivors of the battle were diagnosed with Post-Traumatic Stress Disorder, a phenomenon that came to be dubbed "The Algolis Effect" by members of the Marine Corps.