You're talking as if it's the Ghosts alone are stopping a planetary invasion. Earth hasn't had a planetary invasion. I wouldn't call Twilight Gap a planetary scale invasion when it only involves a couple factions of a race. Not even the Hive landing on Earth is a planetary scale invasion when they're pretty much contained in the Cosmodrone and random outbreaks else where in the world. Only the Cabal and Oryx had the true power to conduct a full on planetary invasion, something that any of the factions in Halo would have no problem with. Or hell, why not just blow up Earth directly? None of the factions of Destiny have that power while all of them in Halo do. I'd love to see The City withstand a NOVA bomb, which is still comparably weak within Halo. You're giving way too much credit to the Ghosts out of blind fanboyism. If they were truly as strong as you make them out to be then The City would be in a much much better position and so would the Guardians, but they are not. The Speaker always says the City is on the verge of collapse and fading into Darkness. Spartan kills the Guardian, Spartan captures or destroys the Ghost. End of story. Spartans have the reflexes and speed in order to do that, not to mention the training and experience.