Well I'll be damned. Someone brought video games to AltHist. Well, Halo's a hobby of mine, and philosophy's a specialty of mine, so I'll see what I can come up with.
The books' leaning would depend primarily on the author's views, because the series' main character, Master Chief Petty Officer John, SPARTAN-117, can be seen in two very different lights. Also, the author would most likely use it as a metaphor for Vietnam, and write it accordingly.
First is the the image the gaming community sees of him: an invincible, alien-slaughtering badass (as one Marine put it in Halo: Evolutions, "An honest-to-Buddha one-man death squad") who is the only thing standing between Mankind and complete annihilation at the hands of marauding aliens. If the books took this viewpoint, the oh-so-subtle message would be that the US military is the paladin of the free world, beating back the evil scourge of those goddam communist North Vietnamese and their Russky allies to save the helpless people of South Vietnam. In this scenario, Reach is the Alamo, the Ark is the Charge of the Light Brigade, and the Chief continues to be a badass with no second thoughts, unwavering in his drive to protect humanity. He most likely ends up nuking the fuckers out of existence and coming home to Earth to get married (to Samus Aran, according to the scuttlebutt) and have insanely badass kids, because that's what happens to triumphant Good Guys.
The alternate image isn't as pretty. If the author's anti-Vietnam, at some point in the series, John is going to realize that Mankind is wrong to fight the Covenant, and instead should try to talk to the San'Shyuum and negotiate for peace. He will also be massively disillusioned when he finds out that he was essentially kidnapped to be a lab rat for experimental augmentation to turn people into emotionless, augmented killers for the same government he's now defending, and was never expected to survive. He will find some kind and understanding (and definitely not xenocidal) Covenant female, who will take him, despite odrers from their respective commanders, to High Charity. The Hierarchs will listen to him, but the USMC won't. So, John will march back into the Sol system at the head of a Covenant armada to overthrow the corrupt, entrenched government and set up a system where both Human and Covenant can live in peace.
Ready, Set, CRITIQUE!!!