grammar - Marine Corps Possessive

I am editing my brother’s paper, and I realized I am unsure about the possessive form of Marine Corps, such as

The best kept secret of the Marine Corps

Is it

the Marine Corps’ best kept secret

or

the Marine Corp’s best kept secret

I am leaning towards the first because the s is part of the word corps.

Answer

As a Marine myself, I can tell you that "Marine Corp's" is completely wrong.

It is technically correct to use either:

Marine Corps's
or
Marine Corps'

If you look hard enough you will find examples of both, and typically one may be preferred. I have never seen either stated as incorrect; one is either more or less preferred than then other.

I personally would stay away from using "Marine Corps's" not only because of personal preference, but because you will get the occasional mispronunciation as "corpses", which, while it is often a Marine's primary mission to turn an enemy into a corpse, we do not like to hear our name mispronounced as such.