When Shepard and the fleets of the rest of the galaxy came through the Sol relay and went on the offensive?
Looking back at some of the cut scene's the scale of the war was shown really well with a co-ordinated effort from all the species fighting together on Earth.
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If the Reapers were truly as great as they said they were, they should have been able to see that Shephard was the protagonist and that they were the antagonist and that antagonists never, ever win. Ever.