under arms

adj — 2 senses

adj

Government, Military, Politics, War Equipped with weapons (of a country, army etc.); in a state of armed, military readiness.

They were preceded by a small party of the national guard, in their uniform, and under arms.1792, Charlotte Smith, Desmond, Broadview, published 2001, page 325:
The Taliban have only about 40,000 men under arms.1998 September 20, New York Times:

Australia Carrying a weapon (chiefly of a criminal); of a crime, committed using a weapon or weapons.

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