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.
As there is no equivalent in the UNSC Marine Corps, Fleet Admirals outrank all Marine Officers (as well as most Navy Officers).
Fleet Admirals typically lead battlegroups charged with the defense of key UNSC planets, such as Earth and, formerly, Reach.
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