The Locust Horde initiated and launched a full-scale, coordinated, worldwide, 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 all at once, 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, suprise assault on a massive scale, every single major population center was attacked and had succumed to the Horde in an instant. And with the Locust Horde managing to either destroy or occupy all major human cities, they killed a quarter of the total human population on the very first day. 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 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 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 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 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 urban, 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.
This is where A.C.I.D. Missions begin.