Geth War

The Geth War, referred to as the Morning War by the geth, was the war with the quarian that occured after the geth developed sentience.

Leading Up to War
The quarians sought to create a cheap labor and war force. They created the geth, an advanced Virtual Intelligence neural network, around 1900 CE. The quarians built more and more geth, and due to their networking ability, they became increasingly smarter. The quarians wanted their geth to be able to complete more complex tasks without supervision. By tweaking the geth's systems, the quarians inadvertently made the geth into an Artificial Intelligence, giving them sentience. The quarians realized their mistake when a geth asked its quarian master whether it was alive and what its purpose was. After the geth became sentient the Admiralty Board ordered an immediate termination of all geth programs.

The War
Once the now sentient geth realized what the quarians were doing, they retaliated. In their initial strike, the geth killed billions of quarians. The geth soon overran the quarian homeworld, Rannoch, and the surviving quarians gathered their ships and fled the planet. They pleaded for help from the Citadel Council, but they denied any help and stripped them of their embassy, punishing the quarians for creating an AI. The geth won the conflict costing the quarians billions of lives, and countless credits.

Aftermath
After the remaining quarians fled the planet, they remained on the ships and created a form of government and even a civilization; it is now known as the Migrant Fleet. The quarian's numbers were devestated, as only about 17 million quarians still alive on the flotilla, and they were still refused an embassy on the Citadel. The geth left Rannoch and fled behind the Perseus Veil, away from the prying eyes of organics. In an attempt to protect the galaxy from the geth, the Citadel sent a fleet to the edge of the Veil. They suspected an imminent attack on the organics by the synthetics, but no attempt was made by the geth to leave the Veil.