A more thorough explanation:

A militia is a group of citizens who are trained and armed for military service, usually by a state. This is separate from the regular armed forces. The Constitution allows states to form a "well-regulated militia," but Congress also has the power to organize and govern a federal militia. The reserve militia includes all individuals who are not exempt from military service and are not actively serving in the armed forces or national guard. In Roman law, militia refers to military service.

• The National Guard is a state militia that can be called upon by the governor to respond to emergencies or disasters.
• The Minutemen were a militia group during the American Revolution who were ready to fight at a moment's notice.
• In some countries, like Switzerland, all able-bodied citizens are required to serve in the militia for a certain period of time.

These examples illustrate how militias can be formed and used in different ways. The National Guard is a modern example of a state militia that is trained and equipped to respond to emergencies within the United States. The Minutemen were a historical example of a citizen militia that played a crucial role in the American Revolution. In some countries, like Switzerland, serving in the militia is a mandatory part of citizenship and national defense.