On marijuana policy, there’s a rift between the federal government and the states. It started with California’s allowing marijuana for medical use in 1996, widened as several other states followed suit, and became too big to ignore 10 months ago, when voters in Colorado and Washington decided to legalize the drug for recreational use. Under federal law possession is still a crime.

After conspicuous silence, the Justice Department announced in August that it wouldn’t try to put the toothpaste back in the tube — it wouldn’t sue to block the Colorado and Washington laws as long as those states put in place “strong and effective regulatory and enforcement systems.” But this policy hasn’t cleared up all the confusion arising from this tricky situation. Many practical questions remain, as became obvious at a Senate Judiciary Committee hearing on Tuesday about conflicts between state and federal marijuana laws.

At the hearing, James Cole, a deputy attorney general, said the Justice Department expects Colorado, Washington and the 18 medical marijuana states to prevent the distribution of the drug to minors, its diversion to states where it is illegal, and its possession or use on federal property, among other restrictions. Senator Charles Grassley, the ranking Republican on the Judiciary Committee, rightly asked how, exactly, the Justice Department would evaluate whether the states were holding up their end of the bargain.

If a 17-year-old was caught smoking a joint in Mesa Verde National Park in Colorado, would federal prosecutors argue that the state wasn’t sufficiently tough on enforcement? Common sense says no, but guidelines are necessary. Mr. Grassley suggested, and Senators Sheldon Whitehouse and Richard Blumenthal, both Democrats, agreed, that the Justice Department should establish clear rules and explain what violations would trigger a crackdown.