User blog comment:GruffMuff/How did the Reapers invade the galaxy in Mass Effect 3?/@comment-1404018-20111122024836

The Reapers were literally heading towards the outer edge of the Milky Way during the events of the Arrival DLC. Supposedly, as Shepard is fighting for his life so that he can call in for the Normandy, from aboard the asteroid station, the Reapers are already entering the system. The local Mass Relay is destroyed, delaying the invasion, but they only have to travel to the nearest system with a relay before they have access to the entire galaxy. Hope this helped,