system
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human
[Halo] Which branch of the UNSC military do the Spartans belong?

So the UNSC millitary is divided into Navy, Army, Marines, Air Force, and the Office of Naval Intelligence.  Which branch of the UNSC military do the Spartans belong? Is it ONI as they were the ones who came up with Spartan project?
gpt
Spartan IIs were under ONI as part of the Navy, Spartan IIIs and IVs were organized as their own branch under United Nations Space Command Defense Force.  
Essentially you have the Army, Navy, Air Force, Marines, and Spartans all as separate branches.