The Halo 3 trailer (which I have watched at least 30 times to date) ends with a mysterious structure (presumably of Forerunner origin) in the middle of an African desert shooting off a blinding beam of light. Followed by the e chord and Cortana stating simply, "This is the way the world ends."
A google for that phrase turns up a poem by T.S. Elliot called "The Hollow Men" which concludes:
"This is the way the world ends
Not with a bang but a whimper."
Could Cortana have been referring to that poem? Has humanity been wiped from the face of the earth at this point? Is there no one to "rage against the dying of the light"?
Only one way to find out…we must finish this fight.