In March, for the first time in the nation’s history, a federal appeals court struck down a gun control law on Second Amendment grounds. Only a few decades ago, the decision would have been unimaginable.

There used to be an almost complete scholarly and judicial consensus that the Second Amendment protects only a collective right of the states to maintain militias. That consensus no longer exists — thanks largely to the work over the last 20 years of several leading liberal law professors, who have come to embrace the view that the Second Amendment protects an individual right to own guns.

In those two decades, breakneck speed by the standards of constitutional law, they have helped to reshape the debate over gun rights in the United States. Their work culminated in the March decision, Parker v. District of Columbia, and it will doubtless play a major role should the case reach the United States Supreme Court.

Laurence H. Tribe, a law professor at Harvard, said he had come to believe that the Second Amendment protected an individual right.