In 2015 the Supreme Judicial Court of Massachusetts, that state's highest court, upheld the criminal conviction of a woman named Jaime Caetano for the crime of possessing a stun gun, which she obtained for purposes of self-defense against her violent and abusive ex-boyfriend. According to the Massachusetts high court, Caetano's conviction must stand because a stun gun "is not the type of weapon that is eligible for Second Amendment protection." Today the U.S. Supreme Court vacated that judgment and ordered the Supreme Judicial Court of Massachusetts to rehear the case.

At issue in Caetano v. Massachusetts is the reach of the Supreme Court's Second Amendment precedents District of Columbia v. Heller (2008) and McDonald v. Chicago (2010), which together recognize that the Second Amendment protects an individual right to keep and bear arms for purposes of self-defense, and that this right applies against legislative enactments by both the federal and state governments.

In its Caetano decision, the Supreme Judicial Court of Massachusetts found that Heller and McDonald were not controlling because stun guns are dangerous and unusual "modern" weapons that "were not in common use at the time of the Second Amendment enactment." Yet as the U.S. Supreme Court observed today, that judgment "is inconsistent with Heller's clear statement that the Second Amendment 'extends…to…arms…that were not in existence at the time of the founding.'" In other words, the Massachusetts high court got Heller wrong and the U.S. Supreme Court just instructed that court to get it right the second time around.

No member of the Supreme Court dissented from today's per curiam opinion in Caetano v. Massachusetts. However, Justice Sameuel Alito, joined by Justice Clarence Thomas, wrote a separate concurring opinion in which he left little doubt that if it were up to him (and Thomas), the state's actions would have been ruled flatly unconstitutional.

"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," Alito wrote. "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." According to Alito, "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 people than about keeping them safe."

The Supreme Court's per curiam opinion in Caetano v. Massachusetts is available here.