When the first English colonists arrived in North America they brought with them an existing militia system developed over centuries.

To be prepared to meet impending threats all men within each county (which usually made up one or more regiments) were required to "muster" or "mobilize" a day or two each year so their names could be entered on the rolls and they could at least familiarize themselves with weapons, usually furnished by the government, stored in a common location called an "armory."

The men were told they were expected to turn out when called for an emergency but in reality they were still civilian farmers or shop keepers and only part-time soldiers.

The most obvious of these were the antiaircraft artillery (AAA) battalions created to defend Washington, D.C., and Hampton Roads from air attack.

Initially armed with guns firing "flak" these were, in the mid-1950s, replaced by Nike missiles capable of destroying enemy aircraft before they could hit their targets.

In the first five years of the 21st century, Department of Defense policy was that no Guardsman would be involuntarily activated for a total of more than 24 months (cumulative) in one six-year enlistment period.