Under the new policy, which James Cole, the deputy attorney general, set out in a memo to federal prosecutors on Aug. 29, the department will not sue to block state laws that legalize marijuana, so long as those states employ “strong and effective regulatory and enforcement systems” that keep marijuana out of the hands of minors, prevent gangs and cartels from profiting from marijuana sales, and punish driving while under the drug’s influence.

The announcement follows closely on Attorney General Eric Holder Jr.’s speech last month on criminal justice reform, which included an order to prosecutors to avoid invoking mandatory minimum sentences when charging low-level drug offenders. Both reforms signal a long-overdue acknowledgment of the costs the nation continues to pay in the indiscriminate, overzealous prosecution of its five-decade war on drugs.

While we believe strongly that states cannot simply pre-empt federal law at their whim, and we are not prepared to support full national legalization of marijuana, there are times when Washington should enforce its prerogatives and times when it should stand aside.

Colorado and Washington both passed ballot initiatives last year legalizing the recreational use of marijuana, but residents and law enforcement authorities in those states had been caught between their laws and the federal law, which treats marijuana as an illegal drug. In addition to Washington and Colorado, 18 states permit the use of marijuana for medicinal purposes, and at least four more are considering similar legislation. The new policy wisely, if implicitly, recognizes this fact.

Public opinion has shifted in recent years. Today about half of Americans, and nearly two-thirds of those under 30, support legalizing marijuana. This may be in part because of the costs and racial disparities in the enforcement of the law. Between 2001 and 2010, there were more than eight million arrests for marijuana possession across the country, costing taxpayers between $2 billion and $6 billion each year, according to an A.C.L.U. report in June. The report also found that blacks are nearly four times as likely to be arrested on charges of marijuana possession as whites, even though both use the drug at similar rates.

The Justice Department’s policy change provides some needed clarity, in particular by ordering prosecutors not to consider a marijuana operation’s size or for-profit nature alone when determining whether to press charges. But the policy remains vague enough that individual prosecutors may continue to go after distributors following state laws, as they did following a similar memo regarding medicinal marijuana in 2009.

In Congress, where it can be difficult for Democrats and Republicans to agree on the time of day, there is increasing bipartisan support for serious criminal justice reform. Two bills now under consideration would reduce many mandatory minimum drug sentences. On Sept. 10, a Senate Judiciary Committee hearing on the conflict between state and federal marijuana laws will take testimony from residents of Colorado and Washington about their states’ experiences with legalization. The nation’s costly and misguided conduct of the war on drugs has not been fixed, but a solution is drawing a little closer.