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After the Secession Wars, both the SDF and UNSA signed the Charon Accords which ended the war and secured peace. However, the SDF took this opportunity to build up and reform its military and space fleets, intending to finally destroy Earth and its legacy at a later date. The SDF also annexed and occupied many Earth off-world colonies, in direct violation of the Charon Accords. Despite these, SATO and UNSA refused to engage the SDF and enforce the Accords, as they were reluctant to engage yet in another war, one they might not win.
The SDF used this to their advantage, attacking supply convoys intended for Earth and backed pirates to attack and raid outposts. They also began to dispute territorial claims, raising tensions and fear that war may spark again. 18 hours before the Geneva Attack, the SDF openly attacked and raided a UNSA Research Facility on Europa. SCAR Team Seven was sent to investigate, destroy the facility and the prototype F-SPAR weapon to prevent it from falling to SDF hands. The SCARs were successful, however, they were captured and executed as the SDF planted their flag on Europa, informally annexing it. Despite the mission result, STRATCOM refused to set the UNSA fleet on a war footing and just made way for the Fleet Day Parade.
6 hours before the attack, Admiral Kotch rallied and gave a speech to his troops, telling them that they will finally eradicate the Earth's influence. This was considered as a declaration of war, but STRATCOM only managed to intercept this rally transmission after the Geneva Attack.
Course of the War
Invasion of Geneva
As SATO and UNSA were celebrating Fleet Day, an SDF sleeper agent took control of the AATIS guns and reprogrammed them to fire on the parade, quickly destroying many unprepared UNSA warships. This also allowed the SDF fleet to jump in and deploy troops, which began to indiscriminately mass murder civilians and military personnel alike while bombarding the already wrecked city. The UNSA, however, managed to quickly regroup its forces and launch a counterattack, with the Eclipse and a ground force of SCAR and Marine forces retaking control of the AATIS guns and captured the sleeper agent, forcing the SDF to retreat into low orbit. The remaining 8 UNSA ships chased and destroyed the fleeing SDF invasion fleet. In a last-ditch attempt to destroy the UNSA Navy, the Olympus Mons jumps out of Slipstream Space, and attacks the already depleted UNSA fleet, obliterating 6 destroyers with its F-SPAR. However, the Retribution initiated a tactical collision maneuver, damaging both ships to the point where the Olympus Mons is forced to retreat. The UNSA is left with just 2 capital ships in operational condition, the destroyer Tigris and the supercarrier Retribution.
Operation Port Armor
During the invasion, SetDef launched an attack on the Moon Gateway Port with resources critical to rebuilding their fleet. Operation Port Armor was launched to take back the port. Troops launched from Retribution and arrived at the gateway and fought through the entire gateway until they managed to retake the port. After the base was secure SCAR Team attack the SDS Ares Vallis and boarded the ship and shut down both primary and secondary weapons while downloading enemy ship positions and killing Captain Bradley Fillion. Soon after they left Ares Vallis and Tigris destroyed it.
Ship Assault Operations
Operation Taken Dagger
The SDS Galaxius had taken possession of a weapon poached off of Europa with kidnapped engineers on board to replicate it. Jackals engaged the ship while SCAR Team 1 and Marines breached the ship. Rescuing the engineers became primary when Captain Radoslav Barkov said that he would kill them if they didn't surrender. SCAR Team 1 soon rescued the engineers in the hanger bay along with killing Barkov and soon moved further into the ship and entered an armory in the missiles bay and got the weapon known as the P-LAW. Then they left the ship and returned to the Retribution.
The SDS Cerberus was carrying a prototype Skelter and SCAR Team 1 and Marines mounted an infill to capture the prototype. They arrived at the ship and shut down the engines making it out of commission. Retribution soon came out of light travel and engaged the ship while SCAR Team 1 and Marines fought their way to the prototype. Then Commander Nick Reyes entered a hanger and killed everyone inside the hanger where the prototype is in including Junior Lieutenant Cesar Magana. After that, he took the prototype and entered the dogfight happening. Several skelter pilots were then killed including Captain Taras Emin, Major Nicholas Rado, and Colonel Young Kim. Then Reyes brought the prototype onto the Retribution along with the rest of the Jackals.
The SDS Hellas was carrying Chemical Weapons and stopped for repairs. SCAR Team 1 and Marines carried out a search and destroy operation to take out the weapons and the ship. During the attempt to reach the ship the Jackals engaged and killed six pilots including Lieutenant Rulon Raymond and Lieutenant Serozh Sarkisyan. They entered a hole in the ship and fought through the ship until they reached the missiles bay and entered an armoury and Reyes stole a sample of the weapons and blew the rest of them up. They then left the ship before it exploded killing everyone in it including Vice Admiral Vlad Derhachov. The Jackals then returned to the Retribution.
Operation Deep Execute
STRATCOM had Intel that SDF commanders Vice Admiral Caleb Thies, Captain Oren Emin, and Captain Khosi Siyada were holding a leadership conference on the SDF Icarus. SCAR Team 1 would take the HIV's full stealth. They waited for a while until the ship along with 3 others jumped out of light travel. Reyes and his partner Nora Salter entered the ship through the drydock hatchway and moved through the ship. Reyes then continued through the ship while Salter took control of comms. Reyes entered a barracks and put on a uniform that belonged to the SDF. Reyes then continued through the ship and killed Tech Officer Negrete and stole his id to use it to enter the room where the meeting was being held. Reyes then entered the meeting room and deactivated the systems in the room, killing all 3 officers. Reyes then followed Junior Lieutenant Andrew Hawryluk while trying to flee and killed him along with several other soldiers firing at Salter. Reyes and Salter fought their way through the ship and fought outside as Retribution appeared out of light travel and engaged the ship. Reyes and Salter then joined the dogfight where several skelter pilots were killed including Lieutenant Matthew Tovar and Senior Lieutenant Omar Gatica. The Jackals then returned to the Retribution.
The UNSA achieved its strategic objective in the war which was to delay or hold off the SDF from assaulting Earth and buy time to rebuild its fleet. However, it came at the cost of losing their entire original fleet, including Tigris and Retribution, the destruction of Earth's Iron Shield and the death of Admiral Raines, although the fate of the STRATCOM staff is currently unknown. The UNSA did recover many resources and facilities from the SDF, enabling them to rebuild their fleet faster than SDF.
The SDF suffered heavy losses, with much of their entire military organization in shambles and their economy in ruins. Several SDF High Council members, which lead the entire SDF in both state and military affairs, were assassinated in UNSA's Operation Deep Execute. Admiral Kotch, their foremost admiral as well as many Ace captains and pilots were also killed throughout the war. The majority of the SDF fleet was also decimated, due to a Trojan Horse-style attack by the captured Olympus Mons on the SDF's Orbital Shipyard on Mars, and the others destroyed by tag-team tactical ops from the crews of Tigris and Retribution. The SDF also lost its primary means of ship production, with the destruction of their Orbital Shipyard and the UNSA taking control of most resource gathering facilities in the Solar System. With neither efficient ship production nor resource production in their hands, the SDF's ability to project interplanetary power was severely depleted.
The war ultimately ends in a strategic victory for the UNSA, with the Earth safe from an SDF invasion, the SDF military and naval forces crippled and with most resource-producing facilities in the Solar System under its command enabling it to quickly rebuild its effectively fleet, unlike the SDF in the immediate future. Despite this, the SDF achieved a tactical victory over the UNSA by decimating its fleet and defensive capabilities. The true end of the war is currently not known for sure, however, the final cutscene in Infinite Warfare, as well as the situation at the end of the game, seems to imply that the UNSA emerged victoriously over the SDF, or at least achieved a stalemate.