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Battle of Psi Serpentis


Battle of Miridem,Operation: WARM BLANKET,Glassing of Alluvion,Battle of Draco III,Battle of Actium,Operation: TORPEDO,Battle of Sargasso,Battle of New Llanelli,Battle of Skopje


Battle of Meridian,Battle of Algolis

Siege of Sirona
Sirona - Groombridge 1618


Human-Covenant War


July 25, 2544 - May 13, 2548




Pyrrhic UNSC Victory

  • Bodua City and Mainland Nature Reserve partially glassed
  • Heavy UNSC and civilian Casualties
  • Heavy infrastructure damage
  • Covenant forces abandoned siege

UNSC Defense Force




Air Force


Marine Corps

  • 170,662 Naval Personnel
    • 65 Groombridge Defense Fleet Vessels
    • 22 Additional Vessel (Under various states of repair)
  • 97,381 Land Personnel
  • 42,850 Air Personnel



  • Moderate terrestrial and exoatmospheric losses
    • About 5 ships
      • About 90% of the fighter complement
      • About 30% of the dropship complement
    • About 20% of ground forces

One of the longest battles in the history of the war (lasting a total of 1389 Earth days), the Siege of Sirona heavily scarred the medical world. Often considered to be one of the worst battles of the war for both sides, more Human military causalities were caused by this single battle than were caused in several other complete glassings of a planet, without even a single city being glassed to completion.


Prior to the siege, the Human-Covenant war had actually seen Sirona develop into a well-defended, major population center. Though the UNSC did not expect an attack on the world, the demand for medical supplies, medical centers away from the front, and repair facilities in a similarly removed location led directly to an increase in Sirona's perceived importance to the war effort. As such, it saw the continued development of the defense increases that had started being implemented following the First Battle of Sirona, with the permanent fleet stationed around Sirona bolstered to 65 vessels, ten orbital weapons platforms similar to those found around Reach constructed in orbit of the planet, and five refit and repair platforms distributed to the world.

However, it was these refit and repair platforms that actually indirectly led to the Covenant discovering the planet and launching the Siege. With recent battles such as the Siege of the Atlas Moons and the Glassing of Alluvion sending ships in need of repair carrying soldiers, marines, and sailors in need of treatment to Sirona, the Covenant managed to place a tracker on one of the last vessels fleeing Alluvion. Although the vessel followed the Cole Protocol and only arrived in the Groombridge System after a series of randomized jumps, this beacon transmitted its location back to the Covenant Hierarchs for analysis. Once the vessel reached Sirona and stopped its random jumps for several days, the Covenant figured that it had come to rest around another UEG world. Thus, the Fleet of Righteous Vigilance was dispatched to the world to investigate and besiege the planet should it prove to be a UNSC stronghold.

However, the UNSC was not caught entirely unaware by this scheme. Approximately a week prior to the Covenant forces arriving around Sirona, a worker assigned to the tracked vessel's repair efforts located the tracker and reported it to her superiors. Unfortunately for the UNSC, the discovery took several days to be recognized for what it was and directed to the UNSC commanders in the area and the response took several days further to see implementation. As such, by the time the Fleet of Righteous Vigilance arrived on the edge of the Groombridge 1618 System on July 25, 2544, it was faced with only a hastily organized defense group of 87 vessels (though 22 were in various states of damage), ten orbital defense platforms, and five refit and repair stations.

The Battle

Orbital Conflict

While outnumbered more than three to one, the Prophet of Conviction saw the disorganized nature of the defending UNSC fleet and pressed a fifth of his fleet forward via and intra-system slipspace jump, hoping to catch the Sironan defenders off-guard. While the plan initially worked and several UNSC vessels were destroyed by the pincer maneuver, Major General Cadiou managed to redirect the fire of Sirona's ODPs quickly, annihilating the five vessels taking part in the surprise attack in turn. Seeing his plan foiled without nearly as much success as he had desired and being notified by his vessel's luminary that a vast sprawl of Forerunner artifacts had been located on the surface, the Prophet of Conviction had his remaining vessels retreat outside of the range of the ODPs and deploy dropship alongside fighter escorts to secure the world from the surface.

Ignoring the human capital ships remaining in orbit of Sirona, these waves of small craft made quick pace for the atmosphere of the world. By bunching large numbers of fighters for each dropship, the majority of the Covenant forces—consisting primarily of Zealots at this stage—managed to run the UNSC blockade successfully, losing only about a third of the dropship compared to almost ninety percent of their fighter escorts. Once past this stage, however, the UNSC Navy (both Space and Wet) and Air Force both deployed fighter craft to pursue the vessels to the surface. However, once again, the Covenant fighters took the majority of the focus off of the dropship, relentlessly engaging the defending forces to keep the humans distracted from shooting down the landing craft.

Once past the last line of serious aerial defense offered by the UNSC, the Covenant dropships split into two groups: one that landed on the capital island in the Epona region and one that landed on the mainland in the Sironan Nature Reserve, the suspected location of the ODP power generation facility. Both forces were quickly met with resistance by the UNSC Army and Marine Corps and, while the forces in the nature reserve managed to hold out much longer, in part due to the proportion of Zealots to standard troops, the forces that landed in Epona—though they did put up a good fight—were quickly wiped off the island by the defenders, barring them from taking out the UNSC leadership and significantly slowing the progress of the other ground campaigns as they now had to be coordinated from orbit.

Siege of Sirona

Covenant frigates bombarding Sirona in atmosphere

The next serious action in orbit of Sirona, as such, only came just under a year after the initial siege. In May 2545, the Zealots in the nature reserve finally captured and shut down the ODP power generating facility. With the main UNSC defensive platforms disabled and the Groombridge fleet whittled down over the past months during failed counterattacks on the besieging Covenant forces, the Fleet of Righteous Vigilance pushed forward and, without losing a single additional ship, destroyed the remaining UNSC vessels, killing Admiral Whitaker in the process and leaving orbital control in the hands of the Covenant forces. At this point, several Covenant frigates descended into the atmosphere to reinvigorate the ground campaign on the planet while the remaining forces remained comfortably in orbit of the world, continuing to block transmissions to the rest of the UNSC and leaving HIGHCOM to assume the world had been lost, even managing to detect and destroy a prowler that had been sent to investigate the world.

At this point, the remainder of the Siege of Sirona was quiet on the orbital front. While additional forces were additionally delivered to and retrieved from the surface to rotate out the besieging army, the destruction of the Groombridge Defense Fleet and the lack of any UNSC reinforcements on their way left there very little to do for the Fleet of Righteous Vigilance except shoot down any transports attempting to flee the world. As such, no additional vessels were lost for the entirety of the siege, with the Covenant eventually deciding to abandon the world in March 2548 under orders of the Covenant Hierarchs to assist, instead, with the Battle of Meridian.

Capital Island Defense

Of the various fronts during the Siege of Sirona, the assault on the planet's Capital lasted the shortest and was the most decisive victory for the UNSC defenders. With the main Covenant forces in Epona landing just outside the region's central city, the UNSC defenders faced a three-pronged attack. Approximately 45% of the landed forces attacked each Epona's central city and began advancing down the west coast of the island for Grannus while the remaining ten percent of the forces split off and advanced across the center of the island, hoping to secure the eastern coast for a pincer maneuver against Epona to claim the region as a staging ground for the Covenant. However, each of these forces met staunch resistance from the defending human forces.

The third group actually seemed to be fairing the best in their mission during the first few days of the conflict. With the group attacking Epona's central city stalled by staunch militia and marine forces working in tandem and the group advancing on Grannus being slowed by constant harassment on their way down the western shore, it was this group that stumbled upon what appeared to be a lightly defended town on the eastern coast of the capital island. Faced with only a trio of militia tanks undergoing a training exercise, this collection of troops—primarily special operations Sangheili—managed to make quick work of two of the vehicles and seemed to be quickly paving a way through the region for the larger reserve of the ten percent to follow through. However, just when they were about to take out the remaining vessels, the Elites found themselves set upon by two platoons of mechanized infantry from the UNSC Army's SPECWAR Group SEVEN. Though these platoons had also been only running training exercises at the time and consisted primarily of soldiers coming off recent injuries, the Elites decided that the forces would be too much to handle on their own and called for their own reinforcements, falling back and maintaining watch over the town as they awaited support.

KSM1 Militia Tank

The KSM1 Jabber tank that saw combat during the well known "Last Stand at Epona"

Delivered by some of the few remaining Covenant dropships in the area over several waves, these reinforcements arrived in short order, consisting of a collection of five wraiths and an additional 150 troops, the remaining forces within the ten percent that had been sent to capture the eastern coast to begin with. In turn, the UNSC call for reinforcements was not expected to be fulfilled for another several hours, leading the defending humans to set up amidst the town and the surrounding hills to hold off the attack from the Covenant. While they did have nine vehicles to the Covenant's five, they were also severely outmanned and outgunned, only sporting about five dozen men, a lightly armed tank, and eight Pangolin APCs to counter the Covenant's capabilities for indirect fire and superior weaponry.

Despite this disadvantage, the UNSC forces managed to use their knowledge of the terrain to their advantage, a factor which—when combined with superior tactical decisions—allowed them to last far longer than the distribution of forces on paper would suggest. Though the Covenant forces managed to destroy all of the defending vehicles and kill the militiamen and soldiers manning them, they did so at the cost of most of their Wraiths and a large contingent of their soldiers. Furthermore, the assault on the town took them far longer than they had wanted, leaving the Covenant in no position to defend against the UNSC reinforcements, even with the additional air support they had received along the way. As such, the UNSC reinforcements were able to quickly regain control of the town, destroy most of the remaining Covenant forces, and secure the eastern coast of the capital island against further advances.

HF Pang 1.9

Two platoons of M874 Pangolins supported a KSM1 Jabber in holding back advancing Covenant Forces during the "Last Stand at Epona"

This effort—referred to as the "Last Stand at Epona" following the conclusion of the Siege—in turn doomed the Covenant forces that were attacking Epona's central city. While they had been holding out fairly well against the human defenders, these forces had gambled on the success of their eastern coast assault to capture the farming community. Having split off a fair portion of their forces to reinforce this attempt—and seeing those forces being destroyed in turn—the Covenant were unable to capture the central city of Epona. Eventually, once the UNSC reinforcements that had instead secured the eastern coast for the humans arrived to reinforce Epona's defenders, these Covenant forces instead started to see themselves pushed back. Unable to retreat or risk leaving the forces attacking Grannus to be caught in a pincer maneuver of the UNSC's making, the Covenant troops in Epona instead fought to the last man, managing to severely weaken the UNSC forces in the process but ultimately unable to defeat the onslaught that had come out of Epona's military bases and up along the eastern coast to meet them.

As such, while weakened, this feared UNSC pincer attack did eventually come to pass. Advancing down the western coast towards Grannus, the remaining UNSC forces from Epona, supported by wet-navy vessels advancing along the shore, surrounded the remaining Covenant forces on the capital island outside of Grannus. Forced once again to fight to the last soldier, the Covenant forces managed to deal moderate damages to the UNSC forces before being wiped out, clearing the Capital of Covenant by September 2544.

Capital Island Defense

A map of Sirona's capital island.

Following Admiral Whitaker's death in May 2545, and with the ODP facilities already overrun, the capital island saw new importance to both the UNSC and Covenant as it became the primary hub for military planning by the remaining defenders on the planet. With Admiral Logan—now taking Admiral Whitaker's command of the naval forces remaining on the planet—and Generals Cadiou, Carro, and Gaume all operating off of the island. As such, the bulk of the remaining air-defense and anti-capital ship forces were withdrawn to the island. Through these combined forces, the UNSC managed to deter any further Covenant assaults on the island for the remainder of the siege, though they still saw forces lost during attempts to reinforce the mainland or evacuate the planet as dropships sent on either mission had a high chance of being shot down. In fact, even General Gaume was killed in such a manner late into the siege when her Pelican was shot down en route to the Mainland in June 2547 during an attempt to transfer command to a location she would be able to better defend the mainland. As such, command of the UNSC Marine Corps forces on the world fell on the shoulders of Colonel Helen Vasquez and the UNSC left further attempts to leave the capital island.

In turn, with the island's facilities comparably untouched by the fighting, the capital island became the primary staging ground during the efforts to clear out the remaining Covenant ground forces following the departure of the Fleet of Righteous Vigilance at the end of the Siege in March 2558, with the planet being declared free of Covenant and contact with the UNSC reestablished properly in May of that year as reported by General Cadiou.

Mainland Defense





July 25 (Fall 27, 420 Local)

  • 0400 Hours (17:00 Local) - Fleet of Righteous Vigilance arrives in the Groombridge 1618 System. Siege of Sirona begins.

July 27 (Fall 29, 420 Local)

  • 0330 Hours (09:00 Local) - "Last Stand at Epona" begins, ending several hours later.
  • 2030 Hours (25:20 Local) - Remaining Covenant forces driven from Epona.

July 28 (Fall 30, 420 Local)

  • 1820 Hours (19:30 Local) - Remaining Covenant forces driven from Grannus, leaving the capital island in the hands of the UNSC.


May 9 (Fall 9, 422 Local)

  • 1725 Hours (25:15 Local) - Covenant Zealots overwhelm defenses at the power generators for Sirona's ODPs. Fleet of Righteous Vigilance takes control of orbital space over the next hours.


May 26 (Spring 8, 425 Local)

  • 1220 Hours (11:45 Local) - Covenant forces breach the defensive line at the ONI research facility of Sirona's Forerunner ruins. Battle for the ruins continues underground.


April 20 (Spring 27, 427 Local)

  • 0715 Hours (09:20 Local) - Maponos Combined Military Installation captured by Covenant forces. Supply lines cut-off for UNSC troops defending the nearby Forerunner ruins.

June 10 (Fall 5, 427 Local)

  • 0845 (07:30 Local) - A radio call goes out over UNSC frequencies that a HQ had been established within Maponos City's Optican building. A group of survivors within Maponos hear the call (including Spartan-II Chief Petty Officer Alaska-013) and meet up, working their way through the city over the next few days.

June 19 (Fall 13, 427 Local)

  • 1745 Hours (09:30 Local) - Last UNSC defenders killed or captured within the Forerunner ruins. The Ruins fall into the hands of the Covenant forces.
  • In the following hours, Brigadier General Suzanne Gaume is killed when her Pelican is shot down making its way to the Mainland to further coordinate the remaining forces.


March 2 (Summer 3, 429 Local)

  • 2030 Hours (15:50 Local) - Fleet of Righteous Vigilance called to the Battle of Meridian, leaving Sirona in UNSC hands.

May 20 (Winter 4, 429 Local)

  • 0000 Hours (22:20 Local) - Last remaining Covenant troops defeated on Sirona, fully reestablishing the planet in the hands of the UNSC.

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