Look at the halo 5 vidocs. They specifically talk about "realizing the Spartan", referencing the books which are canon.
Most halo players even go as far as to say Reach was non canon as it seems to clash with the novel, "Fall of Reach". However, the two can be fit together.
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Reach is non-canon. It does not fit.
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With slight adjustments It can
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There were no spartan 3s anywhere near reach, and they wouldn't have met Halsey at all.
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It's to justify Halo's DBZ story telling method.
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As I already said though, Destiny might be more viable once it's got actual lore.
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