Somebody Please Explain in moderate detail the story line for Halo, Halo 2 and 3?

Hi - this question is coming from somebody who really should know - I used to be pro Halo2 and for Halo 3 im top 2% in the world on Halo Charts - so yea I should understand the game that is classed as my game...lol However I always bought the game for online play - its the only online game I play since 2004 -... show more Hi - this question is coming from somebody who really should know - I used to be pro Halo2 and for Halo 3 im top 2% in the world on Halo Charts - so yea I should understand the game that is classed as my game...lol

However I always bought the game for online play - its the only online game I play since 2004 - and I never really play the campaigns because it just doesnt stimulate me like online play.

I have completed all three campaigns though - but most of the time I had no idea what was going on - I could hardly ever understand what the hell the covenents were saying and so I missed important dialogue and the story line just passed me.

I would appreciate somebody explaining to me the story so far. Oh and please include why Cortana is so dam well important???
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