Contact lenses are widely used and worn by US military personnel.  The
principle restrictions come when you are dealing with full-face
respiratory protection, such as the M-40 or MCU-2/P Chemical Protective
Mask.  The theory being that any particulates that are released from
the carbon filter elements can embed themselves in the eye, rendering
the individual combat-ineffective.  Additionally, if unconscious, the
medics may not know the casualty is wearing contacts.

I find that all bunk.  I am a vet of Operations DESERT SHIELD, DESTER
STORM, NORTHERN WATCH, SOUTHERN WATCH, PROVIDE COMFORT, and NOBLE
EAGLE; I have worn my contacts through sandstorms and other types of
attacks.  I can see much better, and believe my survivability is vastly
improved due to the better vision provided by contact lenses.

Most troops in the CENTCOM AOR now wear 'combat goggles' anyway to
protect their eyes from debris.  Most are ANSI impact-rated, so they
also protect you from some secondary frag.  One of my NCOs eyes were
saved because he was wearing contacts - they allowed him to wear the
old Gargoyle military sunglasses (high-impact resistant.)  We kept them
on the wall after the incident, with a piece of frag sticking through
the lens - never got close to the eyeball...

Anything that provides an individual the ability to wear superior eye
protection (from UV and frag) is far better than the typical GI-issue
traditional glasses (aka "Birth Control Glasses", because chicks will
never get near a guy wearing them...)

Combat vets tend to look at the regs a bit differently than REMFs,
however...