Under arms describes a state of military readiness used to describe regular combat-ready military forces. Military personnel can be generally categorized into two groups: regular forces

Regular army

A regular army consists of the permanent force of a country's army that is maintained under arms during peacetime.Countries that use the term include:*Australian Army*British Army*Canadian Forces, specifically "Regular Force"*Egyptian army*Indian Army...

, or the regular army, and auxiliary forces, or the reserve army. Regular forces are fully trained and fully armed, and are kept in a constant state of readiness for deployment. Auxiliary forces are given sufficient training and are only called up for service during emergencies or during large mobilization

Mobilization

Mobilization is the act of assembling and making both troops and supplies ready for war. The word mobilization was first used, in a military context, in order to describe the preparation of the Prussian army during the 1850s and 1860s. Mobilization theories and techniques have continuously changed...

s. National Guard and military reserve force

Military reserve force

A military reserve force is a military organization composed of citizens of a country who combine a military role or career with a civilian career. They are not normally kept under arms and their main role is to be available to fight when a nation mobilizes for total war or to defend against invasion...

s would be classified as auxiliary forces because the majority of those personnel are usually employed full time in the civilian workforce. The Regular forces are typically not employed in any capacity other than military service, and usually reside on military bases. Therefore, the regulars are considered "under arms" while the auxiliary forces are not. A mobilization consists of the transformation of auxiliary forces into regular forces by physically amassing and arming those personnel.