Complex sentence: Operational from 1940 until 1944, the Home Guard was comprised of 1.5 million local volunteers otherwise ineligible for military service, such as those too young or too old to join the services, or those in reserved occupations-hence the nickname "Dad's Army".
Split into simpler sentences: Operational from 1940 until 1944, the Home Guard--comprising 1.5 million local volunteers otherwise ineligible for military service. These men were either too young to join the services, were in reserved occupations or were too old to join the services-hence the nickname "Dad's Army".