In the United States, state defense forces (SDFs) are military units that operate under the sole authority of a state government. State defense forces are authorized by state and federal law and are under the command of the governor of each state.

The militia of the United States, as defined by the U.S. Congress, has changed over time. During colonial America, all able-bodied men of a certain age range were members of the militia, depending on each colony's rule. Individual towns formed local independent militias for their own defense. The year before the U.S. Constitution was ratified, The Federalist Papers detailed the Founding Fathers' paramount vision of the militia in 1787. The new Constitution empowered Congress to "organize, arm, and discipline" this national military force, leaving significant control in the hands of each state government.