Mr. Sessions is reversing an Obama administration memo that essentially said the federal government would not enforce marijuana laws in states that legalized the drug, provided they included adequate protections to insulate the business from gangs and violence, to keep marijuana out of minors’ hands, and to prevent it from being trafficked over state lines to places where it remains illegal. That reflected a proper focus on the federal government’s real interest in the issue. Before the memo, both during the George W. Bush and Obama administrations, the Drug Enforcement Agency conducted raids on scores of legal medical marijuana businesses that were operating in accordance with state laws and regulations. It was a monumental waste of effort that did nothing to reduce the use of marijuana or to eliminate dangerous drug gangs. Returning to that approach would be a terrible mistake, and leaders of both parties in Congress have been quick to say so since Mr. Sessions’ announced his decision.