National Service

National service is a common name for a system of either compulsory or voluntary government service (usually military service, also known as conscription). The term became standard British usage after the National Service (Armed Forces) Act 1939, and has been applied to such voluntary enrollments as the US Peace Corps and AmeriCorps. Many young people spent one or more years in such programmes. Compulsory military service typically requires all male citizens to enroll for one or two years, usually at age 18 (later for university-level students), while Voluntary national service requires only three months of basic military training. The US equivalent is Selective Service.

...
Wikipedia