It is settled that the Second Amendment protects an individual right to keep and bear arms that applies against both the Federal Government and the States. District of Columbia v. Heller, 554 U. S. 570 (2008); McDonald v. Chicago, 561 U. S. 742 (2010). That right vindicates the “basic right” of “individual self-defense.” Id., at 767; see Heller, supra, at 599, 628. Caetano’s encounter with her violent ex-boyfriend illustrates the connection between those fundamental rights: By arming herself, Caetano was able to protect against a physical threat that restraining orders had proved useless to prevent. And, commendably, she did so by using a weapon that posed little, if any, danger of permanently harming either herself or the father of her children.

Under Massachusetts law, however, Caetano’s mere possession of the stun gun that may have saved her life made her a criminal. See Mass. Gen. Laws, ch. 140, §131J (2014). When police later discovered the weapon, she was arrested, tried, and convicted. The Massachusetts Supreme Judicial Court affirmed the conviction, holding that a stun gun “is not the type of weapon that is eligible for Second Amendment protection” because it was “not in common use at the time of [the Second Amendment’s] enactment.” 470 Mass., at 781, 26 N. E. 3d, at 693.

This reasoning defies our decision in Heller, which rejected as “bordering on the frivolous” the argument “that only those arms in existence in the 18th century are protected by the Second Amendment.” 554 U. S., at 582. The decision below also does a grave disservice to vulnerable individuals like Caetano who must defend themselves because the State will not. . . . .

The lower court’s ill treatment of Heller cannot stand. The reasoning of the Massachusetts court poses a grave threat to the fundamental right of self-defense. The Supreme Judicial Court suggested that Caetano could have simply gotten a firearm to defend herself. 470 Mass., at 783, 26 N. E. 3d, at 695. But the right to bear other weapons is “no answer” to a ban on the possession of protected arms. Heller, 554 U. S., at 629. Moreover, a weapon is an effective means of self-defense only if one is prepared to use it, and it is presumptuous to tell Caetano she should have been ready to shoot the father of her two young children if she wanted to protect herself. Courts should not be in the business of demanding that citizens use more force for self-defense than they are comfortable wielding.

Countless people may have reservations about using deadly force, whether for moral, religious, or emotional reasons—or simply out of fear of killing the wrong person. . . . “Self-defense,” however, “is a basic right.” McDonald, 561 U. S., at 767. I am not prepared to say that a State may force an individual to choose between exercising that right and following her conscience, at least where both can be accommodated by a weapon already in widespread use across the Nation.

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A State’s most basic responsibility is to keep its people safe. The Commonwealth of Massachusetts was either unable or unwilling to do what was necessary to protect Jaime Caetano, so she was forced to protect herself. To make matters worse, the Commonwealth chose to deploy its prosecutorial resources to prosecute and convict her of a criminal offense for arming herself with a nonlethal weapon that may well have saved her life. The Supreme Judicial Court then affirmed her conviction on the flimsiest of grounds. This Court’s grudging per curiam now sends the case back to that same court. And the consequences for Caetano may prove more tragic still, as her conviction likely bars her from ever bearing arms for self defense. . . . If the fundamental right of self-defense does not protect Caetano, then the safety of all Americans is left to the mercy of state authorities who may be more concerned about disarming the people than about keeping them safe.