For the Founders, the militia arose from the posse comitatus, constituting the people as a whole and embodying the Anglo-American idea that the citizenry is the best enforcer of the law. “A militia when properly formed,” wrote Richard Henry Lee in his Letters From the Federal Farmer (1787–1788), “are in fact the people themselves . . . and include all men capable of bearing arms.” From its origins in Britain, the posse comitatus (from medieval Latin meaning “the force of the country”) was generally understood to constitute the constabulary of the shire. When order was threatened, the “shire reeve,” or sheriff, would raise the “hue and cry,” and all citizens who heard it were bound to render assistance in apprehending a criminal or maintaining order. The Framers transferred the power of calling out the militia from local authorities to the Congress.

The Anti-Federalists were not pleased. They wanted the militia to remain under state control as a check on the national government. Many feared that an institution intended for local defense could be dispatched far from home. As Luther Martin objected in Genuine Information (1788),

As it now stands, the Congress will have the power, if they please, to march the whole militia of Maryland to the remotest part of the union, and keep them in service as long as they think proper, without being in any respect dependent upon the government of Maryland for this unlimited exercise of power over its citizens.

In the “Calling Forth” Act of 1792, Congress exercised its powers under the Militia Clause and delegated to the president the authority to call out the militia and issue it orders when invasion appeared imminent or to suppress insurrections. While the act gave the president a relatively free hand in case of invasion, it constrained his authority in the case of insurrections by requiring that a federal judge certify that the civil authority and the posse comitatus were powerless to meet the exigency. The president had also to order the insurgents to disband before he could mobilize the militia. This was the procedure that President George Washington followed during the Whiskey Rebellion of 1794.

Congress authorized the president to federalize the militia in the Militia Act of 1792 (reiterated in 1795):

[W]henever the United States shall be invaded, or be in imminent danger of invasion from any foreign nation or Indian tribe, it shall be lawful for the President of the United States to call forth such number of the militia of the state, or states most convenient to the place of danger, or scene of action, as he may judge necessary to repel such invasion, and to issue his orders for that purpose to such officer or officers of the militia, as he shall think proper.

But even such clear language was insufficient to prevent a challenge to presidential authority during the War of 1812. At the outset of the conflict, President James Madison ordered the governors of Connecticut and Massachusetts to provide militia detachments for the defense of the maritime frontiers of the United States. These governors, however, were Federalists who opposed the war. They claimed that they, not the president, had the authority to determine whether an emergency existed. Governor Caleb Strong of Massachusetts requested an opinion of his state’s Supreme Judicial Court, which concluded that this right was “vested in the commanders in chief of the militia of the several states.” Op. of Justices, 8 Mass. 548 (1812).

The issue was finally resolved by the Supreme Court in 1827 in Martin v. Mott. Although the case explicitly concerned the validity of a court-martial of a militiaman, the decision rendered by Justice Joseph Story validated the claim that the president had the exclusive right to judge whether there was an exigency sufficient for calling forth the militia. State governors, however, retain concurrent authority to call out their respective militias to handle civil and military emergencies, as well as to repel invasions (Article I, Section 10, Clause 3). Houston v. Moore (1820).

Congress’s authority to call out the militia is limited to three purposes: to execute the laws, stamp out domestic insurrections, and defeat foreign invasions. Absent from the list is the ability to call the militia for offensive use in foreign wars. In 1912, Attorney General George W. Wickersham authored an opinion contrasting the army with the militia, arguing that the militia’s service should be domestic in nature. Historically, however, the militia was used across the Canadian border in 1812 and across the Florida border in the Seminole War of 1818.