That was the 1939 U.S. vs. Miller decision where the court essentially reversed earlier rulings regarding the Second Amendment. That case made clear that the militia consisted of “all males physically capable of acting in concert for the common defense” and that “when called for service these men were expected to appear bearing arms supplied by themselves and of the kind in common use at the time.”
In setting forth this definition of the militia, the court implicitly rejected the view that the Second Amendment guarantees a right only to those individuals who are members of the militia.