The Gears of War series takes place on a fictional Earth-like planet named Sera . The planet had a history of conflict that took Sera to the brink of destruction. This shocked the people into a rejection of their destructive ways, leading to a golden age. Although most conflict was solved peacefully, civil liberties and crime remained as troublesome social issues. A golden age of culture, science, and the arts ensued, yet one problem stood prominent above the rest: energy. Sera's sources of petroleum and nuclear fuel were not sufficient to meet the global demand. Imulsion - with its promise of near limitless energy - was thought to be the solution to mankind's great need. Humans discovered liquid Imulsion, which flows underground, and became a main power source. The value of Imulsion led to the Pendulum Wars, in which the Coalition of Ordered Governments and the Union of Independent Republics fought for control of the Imulsion. After 79 years fighting, COG finally defeated UIR by their new weapon, Hammer Of Dawn. Shortly after making peace, however, the humans were attacked across the planet by the Locust, a race of beings that dug their way out from underground. Much of humanity was wiped out during the first Locust attack, otherwise known as "Emergence Day," and the remaining humans gathered their numbers on Jacinto Plateau, its granitic ground is the only place that Locust could not dig through. The humans that could not make it became known as the "Stranded," and took shelter among the ruins. Out of desperation, the COG employed a "scorched earth" policy by firing orbital lasers across the surface of the planet to deny the Locust any spoils of war. The COG forces and the Locust continued to battle, each side desiring the complete destruction of the other.
The campaign mode of the series has focused on the character of Marcus Fenix, a COG soldier formerly imprisoned for abandoning his post during the early Locust attacks to save his father, Adam Fenix, a key scientist studying the Locust. At the start of the first game, Marcus is rescued from an abandoned prison facility by his best friend, Dominic "Dom" Santiago, who had lost his wife Maria during the Locust attacks and has been looking for her since. Marcus, after his rescue, is readmitted to the COG force and promoted to lead Delta Squad. They are later joined by Augustus Cole, a star "thrashball" player before Emergence Day, and Damon Baird, the squad's mechanic. They fight their way through various settings on Sera including the underground world in order to destroy the Hollow, the Locust tunnels, with a weapon called the Lightmass Bomb. In the second game, Delta Squad and the entire COG army launch a massive invasion of the Hollow to destroy the Locust and the Lambent with much of the game taking place in the Hollow.