The Reaper war is a war against total annihilation. It started as a silent war, and without the efforts of John Shepard and his Normandy Crew the war would have been lost before it ever started. Finally once Citadel space recognized the threat, they were able to fight back and defeat the Reapers, thanks largely to John Shepard and his crew. In an all or nothing attempt to destroy the invincible Reapers Shepard and his crew sacrificed a solar system, by luring the Reaper’s into the multi-star solar system and simultaneously detonated the stars destroying everything in the blink of an eye, both planets and Reapers alike. Despite the missions success Shepard has been assumed dead after the Normandy never returned. The citadel has criticized humans greatly for this because although it won the war it sacrificed a innocent species and extensive planetary resources in the explosion.
Essential information on the Reapers: For reasons currently unknown, the Reapers cull the intelligent races of the Milky Way galaxy, returning to dark space with the organics’ technology and resources. The Reapers leave no evidence of their conquest, nor of their existence – only desolate, barren ruins of those who came before. However, due to the events of the ending of Mass Effect 2, it can be considered a possibility that the Reapers use enslaved organic beings in order to create more of their own kind, as seen with the Human-Reaper.
The trap created by the Reapers was simple. A sentient species would develop FTL drive, but would still be limited in its speed. By leaving a network of relays capable of instant transport across the galaxy that led to the impressive Citadel, the Reapers ensured that it would become the center of galactic civilization. Further, Sovereign implies that the presence of the mass relays would lead the sentient species down a predetermined route with regards to weapons and armor technology (both of which are based upon element zero technology for the Citadel races). It is probable that this reduces the possibility of organic life discovering alternative, more advanced technology and progressing down a different ‘path’; or, as Sovereign put it, “We impose order on the chaos of organic evolution.”
Another possibility is that since the Reapers harvest the resources and technology of the organic races, introduction of the Mass Effect technologies (which the Reapers themselves utilize) ensures the organic races will develop technology that will in turn be useful to the Reapers following each harvest.
Once the sentient races have established themselves on the Citadel with the aid of the keepers, an organic race either created or enslaved by the Reapers in order to maintain the Citadel’s basic functions without revealing its secrets, the vanguard sends a signal to the Citadel, instructing the keepers to activate the station’s hidden mass relay. This opens a path between the Citadel and dark space. The Reapers then flood through, killing the leaders of the assembled species before branching out and obliterating all spacefaring life around them.
Because the Reapers first enter the galaxy at the point that they have ensured will be the center of galactic politics, information and finance, they are able to cripple any resistance almost before the Citadel civilizations have any idea that they are under attack. The Citadel also gives them control of the relay network, cutting off star systems from each other and destroying communications.
The Reapers then use their control of the Citadel and its data to begin the most sinister phase of their attack. Records allow them to track down every settled planet and attack them, either stripping the worlds of resources or enslaving the populations with indoctrination and turning them into sleeper agents. The extinction events may be part of the Reaper’s reproductive cycle, in which all the sentient life in the galaxy is harvested and essentially melted down into techno-organic Reaper shells based on the individual species’ physical form.
Once they have harvested the galaxy, the Reapers wipe every trace of their existence from record and retreat back into dark space.