The Obama administration is raising the stakes in a fight over states' rights and firearm ownership by arguing that new pro-gun laws in Montana and Tennessee are invalid. In the last few months, a grass-roots, federalist revolt against Washington, D.C. has begun to spread through states that are home to politically active gun owners. Montana and Tennessee have enacted state laws saying that federal rules do not apply to firearms manufactured entirely within the state, and similar bills are pending in Texas, Alaska, Minnesota, and South Carolina. Yet the Bureau of Alcohol, Tobacco, and Firearms, and Explosives now claims that...