The Home Guard was an armed citizen militia supporting the British Army during the Second World War. Operational from 1940 to 1944, the Home Guard had 1.5 million local volunteers otherwise ineligible for military service, such as those who were too young or too old to join the regular armed services and those in reserved occupations. Excluding those already in the armed services, the civilian police or civil defence, approximately one in five men were volunteers. Their role was to act as a secondary defence force in case of invasion by the forces of Nazi Germany.

A militia is generally an army or some other fighting organization of non-professional and/or part-time soldiers; citizens of a country, or subjects of a state, who may perform military service during a time of need, as opposed to a professional force of regular, full-time military personnel; or, historically, to members of a warrior-nobility class. When acting independently militias are generally unable to hold ground against regular forces; militias commonly support regular troops by skirmishing, holding fortifications, or conducting irregular warfare, instead of undertaking offensive campaigns by themselves. Local civilian laws often limit militias to serve only in their home region, and to serve only for a limited time; this further reduces their use in long military campaigns. Militias may also, however, serve as a pool of available manpower for regular forces to draw from, particularly in emergencies.