- "We could have cooperated with the groundwalkers for our mutual salvation, but they are humans and they only understand dominance and ownership. All that we have left is a war to the death. For all their so-called intelligence, humans are blind to the threat right before their eyes. We never stood a chance of enlisting their aid, so now we fight alone. And we will stand on their corpses to do so."
- — Queen Myrrah on the Locust justification of the genocide of the human race in order to defeat the Lambent Locust
The Locust War, was a seventeen year-long, planet wide conflict between the Serans and Locust races raging on both the surface of Sera and the Hollows. The War began on Emergence Day when the Locust Horde emerged from the depths of Sera and eliminated one quarter of the human population. It took the COG fifteen years to discover the Locust's motivation for the extinction of humanity, when during Operation Hollow Storm they encountered the Lambent Locust, who were also fighting the Locust Horde deep in the Hollows in the Locust Civil War.
The humans who survived continued to struggle for over 15 years after the attack, as they were gradually pushed further and further back. However, in the aftermath of the Lightmass Offensive and the Siege of Jacinto, the Hollows had been devastated from the Lightmass bombing and flooding after the sinking of Jacinto. In the aftermath of Operation: Hollow Storm, the COG was forced off the mainland and settled in the Lesser Islands chain, soon starting a new war with the Stranded and the Lambent invasion of the surface, while the Remnant of the COG and Stranded united to fight the Lambent, the remnants of the Horde continued to fight the Serans and Lambent plague. Most of the Locust degenerated into the Savage Locust, but many still followed Queen Myrrah until the Second Battle of Azura where a weapon created by Adam Fenix destroyed the Locust and the Lambent and ended the war.
Imulsion, a parasitical organism used as fuel by the humans that caused the Pendulum Wars created the Lambent Locust and caused the Locust Civil War. Adam Fenix, aware of the Locust since at least 5 B.E. worked on a solution to the Lambent problem, but it grew too much for the Locust and unwilling to work with the humans and needing to move to the surface to escape to the Lambent, the Locust started a war on humanity. Adam Fenix was aware of the Locust's plans, but was unable to stop them.
Emergence Day and the First 5 YearsEdit
- "Well, there's the war. it's bad, and we're running out of body bags. That's about it, really"
- — Col.Hoffman to his wife.On a day like no other,
The Locust Horde initiated and launched a full-scale, coordinated, planetary invasion of the surface on what would become known as E-Day. The Locust Horde deployed huge numbers of their forces in every major human city, in what was a campaign of rapid dominance. They came right up, directly from the subterranean, in their home, The Hollow. With a scale of rapid domination, coupled with an unprecedented, surprise assault on a massive scale, every single major population center was attacked and had succumbed to the Horde in an instant. And with the Locust Horde managing to either destroy or occupy all major human cities, they managed killed a quarter of the total human population on the very first day. During the Destruction of Halvo Bay, Kilo Squad and Colonel Ezra Loomis managed to kill Locust field commander General Karn, but this was a minor setback for the Locust who continued on undaunted. For the next year, almost all COG nations under Chairman Dalyell were either unable or unwilling to unite in an effort to defeat the Locust Horde, with the fact that ordinary military tactics were unable to repel the Locust, as they could emerge anywhere at any time. When Chairman Dalyell died suddenly of a heart attack, Richard Prescott was hastily elected Chairman in his place. He made one last attempt to unite all the COG nations to launch a counterattack at the heart of the Locust incursion, which was located in southern Tyrus. Despite the need for unity, only the South Islands were willing to contribute to Prescott's plan, with all of the other COG nations only concerned for their own survival.
With Ostri, and the Tyran cities of Jannermont and Andius all falling to the Locust, Prescott decided an asset denial strategy was the only option left for humanity. He enacted the Fortification Act and issued a three day warning, telling all remaining COG forces and essentially everyone across the planet to retreat to the safe haven of Jacinto Plateau, the only part of the planet the Locust could not dig through, before using orbital lasers and chemical weapons to scorch all Locust-infested areas, in order to deprive their foe any spoils of war and victory.  Although aware that most of the remaining human population would not be able to make it, Prescott knew that any delay would only allow the Locust time to react to the COGs plans. Over ninety percent of Sera was destroyed to drive back the Locust temporarily and to deny them all civilian and military resources including food, weapons, vehicles and munitions sites, as well urban, government, military, and manufacturing centers, leaving the planet forever scarred and what's left of the human population homeless.
Despite this almost apocalyptic effort, the Locust survived and soon began to assault COG strongholds and military bases on the Jacinto Plateau. Four years after the Hammer of Dawn counterattack, the Locust breached the Plateau during the Winter of Sorrow. This resulted in the city of Landown falling to the Locust Horde, as the COG failed to protect its supply vessels and to send more Gears to protect the city; which was only a day’s drive away from Jacinto City. The city would be a Locust fortress for ten more years until Operation: Hollow Storm.
The Battle of EphyraEdit
10 years after E-Day, the Locust began a massive assault on Ephyra, the capital of both the COG and the nation of Tyrus. They found a way to dig around the rocks of the Plateau, allowing Emergence Holes to appear across the city and bypassing the COG defenses at the city limits. The Locust forces pushed the Gears all the way back to East Barricade Academy, which was across the river from the main part of the city. Here the remaining COG forces made a last stand effort that was nearly successful. However, Marcus Fenix received a distress call from his father; leading him to abandon his post, taking the targeting laser with him, in an attempt to save his father. This action was carried out at the cost of his fellow Gears and ultimately causing Ephyra to fall. The COG relocated the capital to Jacinto City, at the heart of the Plateau, and the only major human city not to have fallen to the Locust onslaught.
The Lightmass OffensiveEdit
- "They do not understand. They do not know why we wage this war. Why we cannot stop. Will not stop. Why we will fight and fight and fight. Until we win... Or we die. And we are not dead yet."
- — The Locust Queen promising the reemergence of the Horde or extinction.
Seemingly defeated in every major land engagement, the Coalition's only hope was Col.Victor Hoffman’s last ditch operation: the Lightmass Offensive. The plan was to end the war for once and for all. They hoped the Lightmass Bomb would wipe out the entire Locust in the Hollows. The operation began with Col.Hoffman pardoning all the prisoners at the Slab except for Fenix, hoping to bolster the shrinking COG army. At the same time Alpha Squad and Sgt. Kennel's squad were trying to deploy the resonator, while the Lightmass bomb was placed on a train heading for an Imulsion sinkhole.
But the plan soon hit a roadblock when Alpha was ambushed and took heavy losses. Delta Squad was sent to aid Alpha Squad to retrieve the resonator. They took heavy casualties and lost a King Raven. Delta Squad then got data from East Barricade and downloaded it into the Lightmass bomb. They detonated the Lightmass bomb, killing millions of Locust soldiers, and destroying the Outer Hollows. General RAAM, the leader of the Locust Army was also killed by Fenix and Dominic Santiago during the operation.
Return of the HordeEdit
- "They came back, though those bastards came back. Just a few months after the bombing, we know the war with the Locust was only gonna get worse. We had to dig in and keep on fighting."
- — Harper on the reemergence of the Horde
In the aftermath of the Lightmass Offensive, COG forces, believing the Locust to have been wiped out, tried to recuperate; putting half the King Raven fleet out of service for repairs. This meant they were not prepared for any sort of counterattack, and the suddenness of the Locust attack at North Gate led to new phase of the Locust War. Locust forces soon began attacking COG control zones, ambushing a COG patrol at Sovereigns Boulevard and forcing them to abandon North Gate Agricultural Depot. Locust forces began to attack the city of Jilane, one of the main breeding centers of the COG, which caused the local garrison to abandon the city, leaving its inhabitants to their fate. Drones soon began killing off Gear Squads in the Pirnah Badlands, which forced Coalition High Command to send in Delta-One on a Search and Rescue Mission However, this only led to more losses. The Lightmass Bomb had also awakened a Riftworm, one of the Locust deities, which the Horde used to sink small military outpost and other cities around Jacinto Plateau, in order to weaken the structure of Jacinto City. Meanwhile, the COG returned to Jilane to scout for any survivors, only to be attacked by them before being forced to defend themselves from the Locust forces still in the city. As the COG began pulling back to defend their last stronghold, the Locust began frequent raids on various buildings, starting the Siege of Jacinto.
Operation Hollow Storm and the Locust Civil WarEdit
The resurgent Locust soon began sinking cities, with Tollen and Montevado both falling to this new threat. Realizing that even Jacinto was not safe, Col.Hoffman and Chairman Richard Prescott planned a counterattack into the Inner Hollows to attack the Locust in their homes. The plan called for a massive Assault on Landown, which had been under Locust control for 10 years. The soil in Landown was soft enough for Grindlifts to dig through, which would allow Gears to enter the Inner Hollows. The Assault on Landown resulted in heavy causalities for both the COG and Locust Horde, but COG forces were able to hold the city and launch the Grindlifts. During the course of the Operation, the city of Ilima was sunk by the Riftworm, which was subsequently killed by Delta Squad. COG forces also discovered that the Locust had started taking prisoners and torturing them on Beast Barges.
After the Battle of Ilima, Delta Squad was sent on a Mission to New Hope Research Facility, to recover data on a Locust stronghold. After retrieving the data and re-entering the Hollows through Mount Kadar, Delta reached Nexus, the Locust capital. Once they were in the heart of the city, they set off JACKs beacon which allowed a second wave of Grindlifts to deploy into the siege of the city. During the battle, the COG discovered the existence of Locust work camps and the Horde’s motivation for the destruction of humanity; they were in a civil war with their Lambent counterparts and were moving to the surface in order to survive. Their plan was to sink Jacinto City to flood The Hollow and kill the Lambent after escaping the Hollows themselves. Realizing that by altering the timing of Jacinto’s sinking, the COG could destroy the majority of Locust Horde, they began evacuating Jacinto and preparing for its final stand.
Towards the end of the Operation and the near falling of Jacinto city, another Lightmass Bomb was sent via King Raven into the Hollow. After it was knocked out of commission by a Brumak, which subsequently became Lambent, Delta Squad used the Hammer of Dawn to kill it, with the massive explosion that followed taking out the city's remaining support structures, causing Jacinto to sink and sending millions of tons of seawater flooding into the Hollow, drowning most of the Horde. However, the Locust Queen was able to escape the flooding and a message left by Adam Fenix indicates that another threat to humanity survived the flooding. The survivors of the Horde split into two factions: the tribalist Savage Locust and the Queen's Guard, those who followed Queen Myrrah.
In the aftermath of the sinking of Jacinto, the COG army and its citizens headed for the frozen wasteland of Port Farrall, whilst dealing with the last of the Locust. After a few weeks camping in the ruined city, the COG decided it was too inhospitable and began looking for a new place to rebuild humanity. They chose the island of Vectes as it housed a former COG naval base and was on the other side of a marine trench that the Locust could not dig under. They also discovered the islands main town, Pelruan, had a surviving population 3,000 citizens that were still friendly towards the COG. In the following weeks the Gears were forced to fight hostile Stranded pirates who began raiding the area for supplies. One particular encounter lead to over 4,000 more survivors from the former UIR nation of Gorasnaya joining the COG and taking refuge on Vectes.
As weeks passed and the COG began to strengthen the island town into a new stronghold, it began to appear that the Locust Horde had truly been dealt a fatal blow and that the 15 year-long war may finally be at an end. However, as the COG were dealing with the minor conflict of the Stranded Insurgency, a new threat emerged from the Hollows as the Lambent began to invade the surface.
Lambent Pandemic and the End of the WarEdit
- Main article: Lambent Pandemic
18 months after the fall of Jacinto and the flooding of the Inner Hollow, both the Gears and Stranded began encountering Lambent forces on the planet's surface, as well as the final remnants of the Locust Horde. In order to ensure the continued survival of the human race, the COG and Stranded formed a truce in order to fight their mutual, and deadly, enemy. Shortly after, the three factions (COG, Locust, Lambent) engaged each other on the surface of Char, an obliterated area of the planet, scorched by the initial Hammer of Dawn strike orchestrated by the COG in order to safeguard humanity and deny the Locust Horde of their prize.
The intense battle was also the first time the Locust had been seen in large numbers since the brutal attacks on Port Farrall. It is, however, unknown as to where the Locust bided their time during the eighteen months in-between flooding the Hollow and the conflict at Char. It is also unknown how both the Locust and the Lambent escaped the Hollow after the city of Jacinto was 'sunk' by Marcus Fenix and the rest of the Gears. The Locust also built a stronghold on the surface of Sera reminiscent of the Hollow in the Deadlands.
The Locust captured the island of Azura where the COG elite and best scientists were secretly protected after being abducted. The Locust slaughtered everyone but Adam Fenix who had built a weapon that could destroy the Lambent, but they feared its use as it might destroy them as well. The Locust captured Adam so he could refine it so it wouldn't affect the Locust, but he didn't have enough time to do that and stop the Imulsion. When Myrrah learned of Marcus Fenix's plan to reach Azura, rescue his father and use his weapon, she sent everyone she could to try to stop him but failed. During the Second Battle of Azura, Myrrah battled Delta herself on her Tempest, but was defeated by the Hammer of Dawn. The weapon was activated and killed Adam before Myrrah emerged from the Tempest to confront Delta Squad. Myrrah was stabbed to death by Marcus Fenix with Dominic Santiago's commando knife and the weapon destroyed the Lambent and the Locust, ending the war.