Justice Dept.'s new marijuana policy Attorney General Eric Holder. (Saul Loeb/AFP/Getty Images)

It’s official: If you spark a joint in Colorado or Washington State, Attorney General Eric Holder isn’t going to bust down your door.

The Justice Department on Thursday decided to allow those two states to experiment with setting up systems to permit but closely regulate marijuana production, sales and use within state borders. It also instructed its prosecutors that, though growing and using pot is still illegal under federal law, the government should not devote limited resources to small-scale enforcement. From the sound of Justice’s memorandum, the feds won’t even necessarily try to take down big, for-profit marijuana production operations, as long as they comply with state regulations that keep them reasonably under control.

This is the right decision. It offers clarity to states, which have had to wonder how their moves to legalize marijuana would interact with federal law. It doesn’t attempt to waste federal manpower on the labor-intensive enterprise of policing small-time pot use, which should really just be decriminalized anyway. And it will be interesting to see how successfully Colorado and Washington oversee their local pot industries and manage the effects of more widespread use. Their examples could well inspire other states and the federal government to follow quickly. Or they could offer lessons about how states should not deal with marijuana.

True, when Congress prohibited marijuana, it probably did not intend for commercial-scale pot growing to be effectively legal if and when states decided to permit and regulate it. Whenever the executive branch essentially makes new policy by exercising prosecutorial discretion, it should make people uncomfortable. Yet the Obama administration isn’t acting out of simple political or ideological opposition to federal law; it is addressing the reality of Colorado and Washington State’s actions, and the federal government’s limited capacity to enforce the federal marijuana prohibition without state-level policing. So the Justice Department adopted a plausible set of national guidelines for when violations produce consequences that rise to the level worthy of federal attention — instructing prosecutors to crack down on operations that might sell pot to minors, fuel gang activity, move product across state lines or encourage driving while high, for example.

As various states have chipped away at the old marijuana enforcement system — in which locals took care of small-time violations and the feds saw to higher-level concerns — Congress has done little to clarify what the federal role should now be, ceding the federal response to executive branch officials. For the time being, that has resulted in the Justice Department’s pragmatic arrangement, which neither condones the country’s changing attitudes nor quixotically attempts to snuff them out.