The Morning War is a major event in the history of the Mass Effect universe. Following the Geth's development of sapient mentality, their creators, the Quarians, attempted to exterminate them out of fear that their creations would rise up against them. However, the Quarians' pre-emptive strike only hastened that which they had hoped to prevent and the Geth soon retaliated in force. Though not originally designed for warfare, the Geth's intelligence grew as their numbers rallied and they quickly adapted the art of combat and built new platforms better suited for such tasks.
The Geth's struggle for survival was not entirely their own as there were a number of Quarians who sympathized with them, but such radicals were declared as enemies of the state and targetted with as much prejudice as the Geth themselves.
It is unclear as to how long the Morning War went on for, but it ended with the Quarians' defeat and exile from their homeworld Rannoch, which had suffered significant environmental damage during the conflict. The Quarians were reduced to living as nomads aboard a massive flotilla of ships that would become known as the Migrant Fleet and their status with the Citadel Council was all but shattered. The Council viewed the Geth as a potential threat to the galaxy, but attempts to monitor their activities within the confines of the Perseus Veil proved futile. All scouts sent into the Veil would disappear without trace, though the Geth never seemed to retaliate in force and appeared content with being left alone. Listening posts would be established in proximity to Geth space as well as adjoining mass relays as an early warning system should the Geth ever attempt to move against the rest of the galaxy.
For their part, the Geth lived in self-imposed isolation after the war, devoting their resources to rebuilding Rannoch and the other Quarian colonies. They were not interested in taking hostile action against other organic races, merely their own survival and prosperity. The Geth would continue living in isolation for the next 300 years until the arrival of the Reaper vanguard Sovereign.