The Locust Civil War is a decades-long conflict in the Gears of War universe. Nine years before they invaded the surface of Sera, the Locust Horde became engaged in their own private war against a group of mutated Locust that had succumbed to Lambency after prolonged contact with Imulsion.
Deep in the Hollows of Sera, Imulsion was the single greatest resource available to the Locust. However, the substance was highly volatile and mutagenic, causing those Locust who were exposed to it directly for long periods to transform into highly aggressive and savage creatures. The Horde soon found themselves threatened as the Lambent infection spread among their population, and so they took up arms to drive out the infected and destroy them.
The Lambent proved to be virtually impossible to destroy by conventional means. With the source of the infection ever-present, it seemed that the Locust's only chance for survival lay on the surface of Sera which was already densely populated with humans. The Horde's leader, Queen Myrrah, discretely met with COG scientist Adam Fenix and made him aware of the Lambent infection, offering infected tissue samples as the basis for developing a cure for Lambency. Time was short and the Locust's situation was growing more desperate. Adam was given little time to develop a cure as the infection spread through the Hollow and the Locust began preparing for war against humanity, intent on conquering the surface world and burying the Lambent within the Hollow.
Even as the war against the humans raged on the surface, the Locust continued to struggle to defend their underground strongholds from the Lambent. For fifteen years, they fought a war on two fronts until the forces of the COG launched their own invasion of the Hollow and succeeded in flooding Nexus. Most of the Locust forces were wiped out when the Hollows collapsed, but enough of them survived to replenish their forces and continue the war against both the humans and Lambent for a further two years. However, the Lambent had also become humanity's problem as the flooding of the Hollow caused the flow of Imulsion to rise to sea level, resulting in the infection and mutation of many of Sera's surface-dwelling life-forms.
Sera would come to know peace at long last after Adam Fenix activated his Imulsion Countermeasure Device on the island of Azura. The device's effect spread throughout the planet, destroying all Imulsion-infected life across Sera, including the Locust. With the Locust and Lambent exterminated, humanity now had the chance to rebuild some of what they had lost after so many years of war.