Military service is service by an individual or group in an army or other militia, whether as a chosen job or as a result of an involuntary draft.
A militia is generally an army or some other type of fighting unit that is composed of non-professional fighters, citizens of a nation or subjects of a state or government who can be called upon to enter a combat situation, as opposed to a professional force of regular, full-time military personnel, or historically, members of the warrior nobility class.
An army or ground force is a fighting force that fights primarily on land.
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Some nations require a specific amount of military service from every citizen.
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A nation with a fully volunteer military does not normally require mandatory military service from its citizens, unless it is faced with a recruitment crisis during a time of war.
A volunteer military or all-volunteer military is one which derives its manpower from volunteers rather than conscription or mandatory service.