In the Gears of War universe, Sera is the first planet where Humans constructed a permanent colony. Life on Sera was generally rich and full, and the colonists experienced an extensive artistic renaissance resulting in a diverse and happy population, proud of their accomplishments. Unfortunately, the discovery of a fuel-conversion process that turns a local fluid known as " Imulsion" into an inexpensive fuel sends the world economy into turmoil, and leads to a large-scale conflict known as the Pendulum Wars. These wars do eventually come to an end, and peace is briefly restored to Sera.
Unfortunately, the Humans of Sera were unaware that a separate civilization existed deep in the bowels of the planet, which was slowly building a massive force to eradicate the invading Humans. On "Emergence Day," the Locust Horde burst through the ground across every major Human city and began to massacre the populace. The various human governments, caught completely unaware and ill-prepared to combat this sudden attack, were unable to successfully counter the Locust armies, and so rapidly combined all governmental bodies into the Coalition of Ordered Governments, a militaristic autocracy which controlled the remaining human military. With no way to counterattack the Locust forces, they imposed martial law across all human territories and ordered all civilians to retreat to Jacinto Plateau, the location of the last human city. Built upon solid bedrock, the Locust were unable to create emergence holes beneath the streets, and so the city was safe from subterranean attack. After giving the civilians as much time as possible to evacuate, COG forces turned all of their space-based orbital laser weaponry against their own cities, denying the Locust access to conquered Human territories.
The first Gears of War takes place fourteen years after Emergence Day, Gears of War 2 takes place just a year after the events of the first, and Gears of War 3 takes place eighteen months after the events of the second.