Mass Effect: After Armageddon
The Reaper War of 2186 was a short but devastating conflict which involved a race of enormous organic-synthetic hybrids known as the Reapers. Starting with batarian space, the Reapers invaded the Milky Way from dark space and attempted to turn all of its organic life into hybridized husks with no will except to serve their Reaper masters. Palaven and Earth were among the planets hit hardest during the war. The Citadel also suffered severe losses.
Under the leadership of Commander Shepard, the Milky Way species rallied and defeated Reaper forces in a final united push on Reaper-occupied Earth. The entire network of mass effect relays collapsed as a result of the battle. After years of coordinated scientific work, several systems' relays are beginning to reactivate.
Immediately after the war, the highest-ranking surviving representatives of all Milky Way civilizations present at the final battle on Earth signed the Sol Accords, a defensive treaty designed to protect against future attacks from outside the galaxy.