The National Guard is a state-based military force that becomes part of the U.S. military's reserve components of the U.S. Army and the U.S. Air Force when activated for federal missions. It is a military reserve force composed of National Guard military members or units of each state, the territories of Guam, the U.S. Virgin Islands, Puerto Rico, and the District of Columbia, for a total of 54 separate organizations. It is officially created under Congress's Article I, Section 8 enumerated power to "raise and support Armies". All members of the National Guard are also members of the organized militia of the United States as defined by 10 U.S.C. § 246. National Guard units are under the dual control of U.S. state governments and the U.S. federal government.

The majority of National Guard soldiers and airmen hold a civilian job full-time while serving part-time as a National Guard member. These part-time guardsmen are augmented by a full-time cadre of Active Guard & Reserve (AGR) personnel in both the Army National Guard and Air National Guard, plus Army Reserve Technicians in the Army National Guard and Air Reserve Technicians (ART) in the Air National Guard.

The National Guard is a joint activity of the United States Department of Defense (DoD) composed of reserve components of the United States Army and the United States Air Force: the Army National Guard and the Air National Guard, respectively.

Colonial militias were formed during the British colonization of the Americas from the 17th century onward. The first colony-wide militia was formed by Massachusetts in 1636 by merging small, older local units, and several National Guard units can be traced back to this militia. The various colonial militias became state militias when the United States became independent. The title "National Guard" was used in 1824 by some New York State militia units, named after the French National Guard in honor of the Marquis de Lafayette. "National Guard" became a standard nationwide militia title in 1903, and has specifically indicated reserve forces under mixed state and federal control since 1933.

On December 13, 1636, the first militia regiments in North America were organized in Massachusetts. Based upon an order of the Massachusetts Bay Colony's General Court, the colony's militia was organized into three permanent regiments to better defend the colony. Today, the descendants of these first regiments - the 181st Infantry, the 182nd Infantry, the 101st Field Artillery, and the 101st Engineer Battalion of the Massachusetts Army National Guard – share the distinction of being the oldest units in the U.S. military. December 13, 1636, thus marks the beginning of the organized militia, and the birth of the National Guard's oldest organized units is symbolic of the founding of all the state, territory, and District of Columbia militias that collectively make up today's National Guard.

Previous to this, unregulated militias were mustered sporadically in Spanish and English colonies. On September 16, 1565, in the newly established Spanish town of St. Augustine, militia were assigned to guard the expedition's supplies while their leader, Pedro Menéndez de Avilés, took the regular troops north to attack the French settlement at Fort Caroline on the St. Johns River. This Spanish militia tradition and the tradition that was established in England's North American colonies provided the basic nucleus for Colonial defense in the New World.

The militia tradition continued with the New World's first permanent English settlements. Jamestown Colony (established in 1607) and Plymouth Colony (established in 1620) both had militia forces, which initially consisted of every able-bodied adult male. By the mid-1600s every town had at least one militia company (usually commanded by an officer with the rank of captain), and the militia companies of a county formed a regiment (usually commanded by an officer with the rank of major in the 1600s or a colonel in the 1700s). The first federal laws regulating the militia were the Militia acts of 1792.

From the nation's founding through the early 1900s, the United States maintained only a minimal army and relied on state militias, directly related to the earlier Colonial militias to supply the majority of its troops. As a result of the Spanish–American War, Congress was called upon to reform and regulate state militias' training and qualification.