Under arms

Under arms describes a state of military readiness (actual or ceremonial). Typically, troops are considered "under arms" when they are in uniform, on duty, and carrying a weapon (rifle, side-arm, or sword), as opposed to being in uniform, on duty, but not carrying a weapon.

Soldiers not actually carrying a weapon but wearing a service belt or web gear associated with carrying weapons may be considered to be symbolically "under arms".^[1]

Soldiers normally remove headgear while indoors; however, soldiers who are under arms normally wear their headgear indoors,^[2] with a few exceptions.^[3]

References

1. Byrd, Roy R. (2003-03-31). "Marine Corps Uniform Regulations" (PDF). United States Marine Corps. pp. 7–10. Retrieved 2013-08-02. The leather service belt is the standard sword belt for officers and may be issued to SNCOs and NCOs for ceremonies, parades, honor guards, and reviews when the sword is prescribed, and upon other occasions when an individual is actually or symbolically 'under arms.'
2. Neptun, Daniel A. (February 2009). "Uniform Regulations" (PDF). United States Coast Guard. p. 2-1. Retrieved 2013-08-02. [Headgear s]hould not be worn indoors or in no-cover areas unless under arms
3. "Drill and Ceremonies" (PDF). United States Department of the Army. 2012-01-20. p. A-3. Retrieved 2013-08-02. Officers and enlisted men under arms uncover only when ... Seated as a member of (or in attendance on) a court or board [or] Entering places of divine worship [or] In attendance at an official reception.