In a major victory for gun rights advocates—and the people of this country—the U.S. Supreme Court ruled 5-4 Thursday that the ownership of firearms is an individual right not tied to service in a state militia.

It was the first time the court had decisively ruled on the question.

The Second Amendment reads: “A well regulated militia, being necessary to the security of a free state, the right of the people to keep and bear arms, shall not be infringed.”

For years, those opposed to private gun ownership have argued that there is no individual right to bear arm...