With a McGuffin like in every other scifi series or film where you fight against impossible odds of course or alternatively killing the head of the evil empire.
Funny thing is the evil saucier ehm reaperboss was pretty close all the time.
Would be funny watching the Reapers arguing and bickering like the council after starkid got purged from the Citadel systems like annoying malware.
Harbinger: Assuming direct control of this war effort!
Reaper number 12755: Dude, you already screwed up with the collectors.
Reaper 3492: Yeah and you killed my progenitor race and this solution is stupid anyways. 
Reaper 2000: I am a nation independant, free of starbrat control and we have no interest in getting killed. These organics actually shoot back!
Reapie 3: What makes you boss anyways?
Harbinger: I am the first made from the apex race and you will follow my command!
Reapie 3: Yeah sure, you are and were a jerk. heating up main cannon
You could have won convewntionally with more preparation probably. You don´t get reliable data about how many dreadnoughts are needed to shoot down a giant cuttlefish with or without Thanix if you didn´t kill some of them. Or that a cruiser can take on a Reaper destroyer.
This stupid council.
Or wait a moment, they are just envoys.
Perhaps the asarian/human taxpayer or the dalatrasses never wanted to pay wartime taxes because someone had a dream that evil squids will come from outer space.
Back to the question at hand. Not many people complained about the existence of a magic mcguffin known as the crucible, it was more about what this thing actually does.