With the release of Mass Effect 3 just weeks away I figured I would add my two cents on what is too come in the next game. Shepard and humanity will be pushed to the brink to defeat an enemy who slaughters millions without hesitation. It would seem the Reapers are a seemingly invicible enemy but are they?
Sovereigns destruction in Mass Effect One showed that the Reapers can be killed. Even though it took an entire fleet to destroy one Reaper it proved they can be killed. I believe the Reapers weakness exists in their overconfidence in their success which for millions of years has proved right so far. The Reapers have wiped out all previous galactic life and civilizations such as the Protheans and despite their best efforts to fight back have proved utterly futile. But here are some mistakes that could lead to their downfall in Mass Effect 3.
One: The Prothean Beacons on Ilos and Eden Prime demostrated to Shepard that the Reaper threat was real. Despite the Protheans being wiped out and turned into the Collectors, they left enough of a warning to future generations to be forewarned of their arrival. Because of this Shepard acted on the information that ultimately led to Sovereigns defeat at the Citadel. Plus the threat of the Reapers was confirmed when Shepard spoke to Sovereign on Virmire.
Two: With the knowledge of the Reapers, Shepard has delayed their arrival at least by a few years. Before when the Reapers wiped out the Protheans they never saw it coming. The Reapers were able to destroy the leaders of the Prothean Empire at the Citadel and cut off Prothean systems from one another to ensure they couldn't communicate or travel by using the Mass Relays. But with Sovereigns destruction at the Citadel and the Council saved, this delayed the Reapers arrival and ensured that the Mass Relays could still be used. Also Vigils information allowing Shepard to have access to the Citadels controls made sure the Keepers couldn't help the Reapers return. There is no way the Reapers anticipated that their plans would be set back so far just by the actions of one individual. Plus when Shepard destroyed the Collectors and also the Alpha Relay from the Arrival dlc it showed the Reapers that the enemy they were facing would go to any length to stop them from returning. Even with the Council dragging its heels on the Reaper threat Shepard took the initiative to fight back against the Reapers and delay them. Shepard became an unknown quantity to the Reapers and even though his actions demostrated to the Reapers he would fight they still don't know what he has planned.
Three: The lack of respect for organic life means the Reapers overconfidence in their victory almost doomed to fail. In a war you should always respect your enemy and expect the unexpected. These two things the Reapers failed on both accounts. With their lack of foresight into expecting that there would be resistance to them and their return they allowed one man to virtually hinder their otherwise timely arrival. One Reaper clearly wasn't enough given that Sovereign was so easily destroyed once Shepard took control of the Citadel back. Because of their lack of respect for organic life they made the mistake into thinking that their would be know way for them to be defeated. The Protheans ensured that future civilizations would find their artifacts and learn about the Reapers arrival and what happened to the Protheans. The Reapers underestimated organics and didn't see that the Protheans ensured their downfall with leaving the Beacon on Eden Prime and Vigil on Ilos. Also from what has been revealed through the Mass Effect 3 demo and information leaks, the Protheans developed some kind of weapon to be used against the Reapers and defeat them. This is why Shepard in Mass Effect 3 travels to the various worlds and recruits the different species to fight the Reapers.
Four: Isolation is one of the most powerful weapons in the Reaper arsenal. With it they cut off the Prothean systems from one another and were able to defeat them. But with their underestimation of organic life the Protheans ensured that the future civilizations would have a fighting chance. And given Shepard acted on the information Sovereign was destroyed and the arrival delayed with the destruction of an entire Batarian system. Because of this the Mass Relays are still linked and the galaxy can better defend itself from the Reapers. Because of Shepards mission to gather the military forces of the galaxies against the Reapers, the Reapers will have a harder time fighting back. Because they no longer face an enemy cut off from help but instead face an enemy who will be unified to stop the Reapers. This more than anything will ensure Shepards victory in Mass Effect 3.
Don't get me wrong it won't be an easy fight and the Reapers are a powerful enemy but with what I listed above I think the galaxy has a better chance of ending the cycle once and for all.