Fleet admiral
From Halopedia, the Halo wiki
Fleet Admiral is the highest commissioned officer rank of the UNSC Navy, although promotion to this rank is restricted to times of war. It is equal in rank to General of the Army. They are generally well experienced and battle hardened naval commanders.
As there is no equivalent in the UNSC Marine Corps, Fleet Admirals outrank all United Nations Space Command Marine Officers (as well as all other Navy Officers).
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