I've had this debate on a few other forums, I know it can seem weird for non die hard fans ahah! I think the main clincher is how humanity are at war with the Covenant again. Obviously everybody who has played Halo 3 would know this makes entirely no sense, as the series ends with a symbolic handshake between Terrance Hood and the Arbiter. There's been some weird theory floating about that you're only fighting a Covenant splinter group, however me and a few guys looked into this, there's literally no reference to this in game!
We have to assume Halo 2 and 3 never happened!
