They say that the covenant didnt expect humanity to be at earth, which is why the fleet they attacked with was so small, they went there for another reason, one that Regret obviously went down to the planet for. it could be another artifact link, im not sure.
There is a reference to New Mombasa in I love bees but its just a passing reference. What annoyed me was the way Halo 2 contradicted "First Strike", First Strike established that at Unyeilding Heidrophant (sp?) the covenant were building a massive arsed fleet to goto earth with, which would explain why the fleet is so small when it arives, however, cortana should have known that because she was part of the assault on it.
So after getting their fleet of 500-odd ships and the Uneven Elephant kicked hard in the backside by Cossack admiral with Texan drawl and a junior lieutenant, the Covenants just shrug and decide to invade Earth half-assed with whatever remnants of that fleet anyway?
And how does the Covenant social structure work? not their military structure, their social one. Do they know economy at all? why do they have all that massive fleets and millions of soldiers with no dangerous enemy to face? (before they met humanity and Flood, I mean)
I know they keep expanding and converting races, but their gigantic military machine is still a bit of an overkill, isn't it? who pays for all the stuffs? who MAKES the stuff?
And considering assuming that Cortana had some sense to steal Covenant techs from Ascendant Justice, and perhaps even kidnap some of its Engineers, it's very possible that the war can start turn around - seeing how humans can wield the Covenant technologies even better than the Covenants themselves.