User blog comment:Martolives/Mass Relays/@comment-2079715-20120402103043/@comment-2076032-20120402104130

Control of the Citadel means control of the mass relay network. The keepers normally handled this, but their signal was detected and disrupted by the Protheans. There is probably a relay on the other side of the Citadel, out in darkspace, where the Reapers reside. This would allow instantaneous access, but this was prevented. So Sovereign set Saren into motion.

Upon Sovereign's failure, the Reapers obviously decided to advance on the Milky Way. This advance took over two years - roughly the length of time between ME1 and 2. By Arrival, the Reapers are only two days away from the edge of the galaxy, and the Viper nebula, where the Alpha Relay resides. This relay is destroyed

It then takes the Reapers another six months to reach Earth. These are not unreasonable time frames without relays for what is probably very advanced Reaper FTL technology, but with the relays, it would have been instantaneous. If the Protheans hadn't have messed with the Keepers, there would be no war. There would be no ME1. The various colonies would be isolated from one another, and the Reapers would jump from one to the next and dominate them.