Firstly the way Reach fell. Reach has always been pushed as the center of the UNSC's military strength, and yet the planet was held hostage by a SINGLE Covenant Super Carrier? Furthermore, the war has been waging for what, 20+ years? (can't be assed to look it up at the moment) The idea that the UNSC hasn't found a way to counter Covenant space stealth is near moronic to me.
This part annoys me especially as Eric Nylund's Fall of Reach painted the picture of a desperate and gritty planet wide genocide.