The concept of the citizens’ militia, as protected by the Second Amendment, is an informal assembly of able-bodied, ordinary citizens acting in concert for the security of our nation. Heller, 554 U.S., at 600 (“citizens’ militia” is a safeguard against tyranny). “[T]he Militia comprised all males physically capable of acting in concert for the common defense.” Heller, 554 U.S., at 595. There are at least two reasons why the militia is thought to be necessary to the security of a free country. First, it is useful in repelling invasions. Second, “when the able-bodied men of a nation are trained in arms and organized, they are better able to resist tyranny.” Heller, 554 U.S., at 597–98. For service in the citizens’ militia, one is expected to bring for action a commonly used firearm such as a gun used for self-defense at home or for hunting game. “Ordinarily when called for militia service able-bodied men were expected to appear bearing arms supplied by themselves and of the kind in common use at the time.” Miller, 307 U.S., at 179. “[W]eapons used by militiamen and weapons used in defense of person and home were one and the same.” Heller, 554 U.S., at 624–25 (citation omitted).