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The UN-MCR War was a conflict fought in the Sol System by forces of the United Nations Navy and Martian Congressional Republic Navy, serving as the culmination of decades of tension building up in the Cold War and a conspiracy conducted by Protogen to cause a conflict. The fighting centered primarily in the Outer Planets - notably the Jupiter planetary system, though fighting was also conducted amongst the Inner Planets. The war saw the destruction of dozens of military spacecraft, the near-collapse of Ganymede Station, the destruction of a city on Earth and several million civilian casualties, alongside the removal of Esteban Sorrento-Gillis and Sadavir Errinwright from power in the UN. The UN-MCR war was the first major interplanetary war fought by humanity.

Prelude to warEdit

Full article:UN-MCR Cold War

The UN and MCRN had been mutual superpowers in the Solar System for decades prior to the war, with tensions escalating constantly. However, these tensions came to a head following the destruction of the Canterbury and the sequence of events that followed. In the space of a few weeks, the MCRN flagship Donnager would be destroyed by an unknown class of stealth warship, which Mars and the Outer Planets Alliance believed were constructed by the UN. In reality, however, these events were orchestrated by a conspiracy involving Jules-Pierre Mao, Sadavir Errinwright and Pyotr Korshunov following the discovery of the Protomolecule on the moon Phoebe. The Eros Incident would cause Earth to launch half of its ground-based missile fleet in an attempt to stop the asteroid colliding with the planet, though the missiles would end up being diverted off-course, with the majority being self-destructed and a handful being stolen by Fred Johnson and the OPA.

Following the Eros Incident, MCR and UN ships would be dispatched to Venus to investigate the Eros crash site, while the MCRN Scirocco and UNN Nathan Hale would be dispatched to Phoebe, the latter a response to the deployment of the Scirocco. The Hale would be deployed alongside the widespread deployment of UNN forces to their bases in the outer planets, as to prevent the deployment of a warship to Phoebe from looking suspicious. Ultimately, however, the Scirocco would use its missile payload to destroy the moon to prevent the UN from taking control of the moon. In response, the UN would use several missiles to destroy the Martian moon Deimos, neutralising the MCR's deep-radar station there. The aftermath of this escalation would see heightened tensions between the United Nations and Martian Republic.

Ganymede IncidentEdit

Full article: Ganymede Incident


CQB combat over Ganymede

Following Phoebe's destruction, the Scirocco would be sent to the Jovian moon Ganymede to help secure the MCR's position there. A Protomolecule Hybrid would be released onto the surface of the moon by Project Caliban to prove the supersoldier's strength in fighting UN Marines and MCR Marines. The confusion caused by this skirmish would ignite the already-heightened tensions around the moon, leading to an orbital and surface engagement that would leave the MCRN in control of the moon and only one survivor on the ground; Gunnery Sergeant Bobbie Draper. For several weeks, the MCRN would control the moon as refugees fled the collapsing Ganymede Station and aid ships attempted to help the station rebuild, though these efforts would eventually be impeded due to the MCRN's implemention of a no-fly zone around Ganymede with authorisation to shoot down anything that interfered with this order while peace talks between the UN and MCR were in progress back on Earth. However, these negotiations would be simply a distraction implemented by Korshunov to buy time for the MCRN Karakum to land on Ganymede and take control of Project Caliban materials. The ship would be shot down by UN forces acting on the orders of Errinwright before it could reach its destination, while Korshunov was simultaneously assassinated to ensure that the only ones who would be able to secure the Protomolecule as a weapon would be the UN. These events would lead to the re-ignition of fighting in the Jovian system, with the conflict eventually spreading across the system. To further inflame the situation, Sergeant Draper would defect to the UN upon discovering the MCR's involvement in the Protomolecule conspiracy.

Declaration of WarEdit

War would be officially declared by the United Nations following the renewed fighting around Ganymede. Before this relations would further deteriorate with death of MCR defense minister Pyotr Korshunov with the Martian Congressional Republic acused the United Nations of assassinating[1]. With the UNN Jupiter Fleet engaged around Ganymede, the UNN 5th Fleet was deployed in a hard burn to reinforce the 3rd Fleet, though the MCRN were succesfully able to destroy ten UNN ships, forcing two to be scuttled and leaving seven to stay engaged with the MCRN.[2]

Bringing the conflict to the home frontEdit

Full article:Nuclear strike on earth


Nuclear missile deploying warheads at Earth

With the UN 5:1 ship advantage rapidly reduced to 3:1, the UN began seeking alternate means of reducing the MCR's ability to fight - a lucky discovery by UN spy satellite Watchtower-14 was able to provide the UN with the potential locations of five MCRN Stealth C Ballistic Missile Platforms. With each platform capable of carrying twenty planet-buster missiles, each loaded with several powerful warheads, the UN were presented with the opportunity to remove the MCR's first-strike ability to launch planet-killing missiles at Earth. Due to the automated nature of the platforms, the strike had to be carefully timed to ensure that each platform was destroyed simultaneously, otherwise the automated system would automatically launch missiles at Earth.

Initial estimates on the success of the strike were placed at 82%, which Secretary-General Sorento-Gillis deemed too low and ordered a cancellation of the strike. Later estimates would up the certainty of a successful attack, leading to the Secretary-General to authorise the strike; however, a malfunction would occur in Rail Gun Turret five, leading to the gun firing after its intended time - this delay of a few seconds would provide enough time for the stealth platform to let off one missile before its destruction. Although Earth's planetary defense grid would be able to destroy the majority of warheads, one was able to successfully evade the defense grid and impact in South America, killing two million people in the immediate explosion.

Pursuit of AvasaralaEdit

Full article:Attempted UN asassination of Chrisjen Avasarala


Preparing to storm the bridge

To cover up his involvement in Project Caliban and the Eros crisis, Errinwright would accuse Chrisjen Avasarala of being complicit in the conspiracy to weaponise the Protomolecule, leading to her being branded a traitor. At the outbreak of the war, Avasarala was aboard the Guanshiyin, meeting with Jules-Pierre Mao to discuss the Protomolecule project. Upon discovering the news of Korshunov's assassination and the destruction of the Karakun, Mao would evacuate the yacht aboard a shuttle headed for Io while a Leonidas-class battleship was sent to destroy the Guanshyin. Avasarala and Bobbie would escape aboard the racing pinnace Razorback while Cotyar Ghazi would escape in a Dropship. Realising that the UN had turned on her, Avasarala broadcasted a distress message on MCR channels, preferring imprisonment by the enemy over execution, that would be picked up by the crew of the Rocinante. The formerly-MCRN frigate would use its remaining munition supplies defending the Razorback from the battleship in pursuit while Cotyar's dropship would be picked up by the UNN Agatha King, under command of Admiral Souther.

Upon hearing the news that Avasarala's personal spy had been captured aboard the Agatha King, Admiral Augusto Nguyễn would personally venture onto the ship to interrogate the prisoner. The King, now under Nguyễn's command, would then be diverted to Io, to take control of the Protomolecule project.

Raid on Kittur ChennammaEdit


Rocinante approaching the K.C.

While en route to Io, the Rocinante would come across the wreck of the Morrigan-class patrol destroyer MCRN Kittur Chennamma, destroyed in battle with UNN forces. The Roci's crew would board the wreck in an attempt to secure fuel and supplies to restock their PDCs and torpedoes. The Roci crew would rescue three MCRN survivors from the Kittur, who would attempt to commandeer the frigate to be put to use in the war, though would fail in their attempt. The crew of the Rocinante would then partially repair the Kittur to allow the MCRN personnel to return to their fleet, with Avasarala providing them with a copy of Errinwright's message to give to MCR command with the aim of relaying it to Admiral Souther.

Mutiny aboard the Agatha King and end of the warEdit

Full article:Io Campaign


UNN Agatha King engaged in combat with ships from the UNN Third Fleet over Io following the destruction of the UNN Jimenez

With MCRN forces pulling back from Saturn to regroup at their shipyards on Callisto, UNN fleets including the 3rd, 5th and 8th were converging on the moon in what was looking to be the single-largest fleet action in history. MCRN 3rd Fleet was able to deal significant damage to the UNN fleets, but at the cost of four battlegroups, leaving the UNN with nineteen Destroyers heading to the moon, for a total of fifty-one UNN Destroyers facing off against the MCRN's remaining twenty-three.[3] Upon learning that the Agatha King was headed for Io instead of Callisto as would be expected, the MCRN Hammurabi diverted its course away from Callisto to intercept the UNN flagship. With the cruiser aiming to intercept the Agatha King, Admiral Nguyễn ordered the UNN fleet to instead rally at Io. Although the Hammurabi was merely a cruiser facing off against a dreadnought, the superior technology of Mars meant that it would be able to fire at the Agatha King long before the battleship could fire back, giving the cruiser a slight advantage were it to come to combat. Upon arriving at Io, Nguyễn was able to transfer control of the Hybrid pods' launch mechanisms - housed in a facility on the surface - to the Agatha King.

With Nguyễn keeping the crew of the Agatha King - including Admiral Souther - in the dark about why they were heading to Io and removing Souther from the Combat Information Center (CIC), the crew of the King began to suggest that Souther forcibly take back command of the fleet. However, it would not be until Captain Sandrine Kirino of the Hammurabi decided to relay Errinwright's message to Souther that Souther was persuaded to do so, leading a general mutiny against Nguyễn for fighting an illegal war. Nguyễn was shot in the chest, though was able to kill Souther in the midst of his broadcast announcing the mutiny. With the UNN ships loyal to Souther beginning to fire on the ones loyal to Nguyễn, Captain Kirino would announce that any ship who broadcasted peaceful intentions would be considered to be flying under a flag of truce with Mars. Now-heavily injured, Nguyễn ordered a launch of the Hybrid pods from Io, with the intent of spreading the Protomolecule to Mars. Many pods were shot down by the Rocinante and the joint-fleet above Io, though many more were able to escape the Jovian system and begin heading sun-ward.

One of these pods would impact the Agatha King, spreading Protomolecule onto the ship, infecting Cotyar in the process. Knowing he was doomed, the spy would begin to cause a reactor meltdown of the ship to prevent the contagion from escaping - this would, however, mean that there would be no way to regain launch control of the Hybrid pods heading for Mars. Ultimately, the crew of the Rocinante were able to land on Io and capture or kill everyone involved in Project Caliban, eventually destroying the site entirely with a nuclear weapon. The pods would be intercepted and destroyed thanks to Fred Johnson, utilising the missiles he stole from Earth in the Eros Incident to shoot down the pods before they were able to hit Mars. With the destruction of Agatha King and the ousting of Errinwright for treason back on Earth, the UN-MCR War ended in time to observe the Protomolecule on Venus rise out of the planet's atmosphere and form the Ring Station at the edge of the Solar System.



  1. The Expanse TV series: Season 3, episode 1 - Fight or Flight
  2. The Expanse TV series: Season 3, episode 2 - IFF
  3. The Expanse TV series: Season 3, episode 5 - Triple Point