Here’s a look back at the range of comments Jeff Sessions has made about marijuana during his time as a senator and now as attorney general. (Joyce Koh/The Washington Post)

Attorney General Jeff Sessions is pushing the federal government back into marijuana enforcement. This is an unwise and unnecessary move that may divert resources from more serious problems — and end up backfiring on those who want to restrain pot use.

Mr. Sessions rescinded Thursday a policy that kept the federal government largely out of the way of states that have legalized marijuana. A majority of states have now legalized it in some form. Maryland just began permitting medical marijuana. California just legalized recreational marijuana, and Vermont is near to doing so.

Mr. Sessions’s move upended a tenuous deal the Obama administration made with legalization states: keep pot out of minors’ hands and help combat trafficking, and federal authorities will focus on bigger priorities. This policy allowed a handful of states room to experiment with unencumbered legalization, which would have made the consequences clearer to others.

Mr. Sessions’s decision is unlikely to result in arrests of small-time marijuana users. But it will chill the growth of the aboveboard weed economy by deterring banks and other institutions from participating. From there, U.S. attorneys across the country will decide whether to crack down, and on whom — a few big distributors, perhaps, or a few local grow shops, too. In states with complex regulations on marijuana growing, testing and selling, some operations may move back underground rather than provide documentation to state authorities that federal prosecutors might later use against them.

Mr. Sessions’s move is counterproductive even for skeptics of legalization, whose only defense against a growing tide of public opinion would be evidence that full legalization has significant negative consequences. Mr. Sessions’s move diminishes the possibility of drawing lessons — including cautionary ones — from the examples of legalization states. Similarly, Mr. Sessions has made it harder to learn how to regulate the legitimate weed economy, if that is the path the country chooses.

Jars of medical marijuana are on display on the counter of Western Caregivers Medical marijuana dispensary in Los Angeles. (Richard Vogel/Associated Press)

More concerning is the prospect that U.S. attorneys will begin diverting limited federal resources into anti-pot campaigns from far more pressing matters. As Mr. Sessions himself said this past November, the nation is experiencing “the deadliest drug crisis in American history.” That would be the opioid epidemic, which, Mr. Sessions noted, claimed some 64,000 lives in 2016. Marijuana simply does not pose the same threat, and the attorney general should have avoided any suggestion that it requires more attention right now.

Mr. Sessions’s decision will spur calls for Congress to finally change federal law. That is warranted, but lawmakers should be wary of swinging too far in the opposite direction. As a recent National Academies of Science review found, experts still know relatively little about marijuana’s health effects. It makes no sense to lock up small-time marijuana users, but it may not make sense to move quickly to national legalization. Rather, Congress should decriminalize marijuana use, then await more information.