Attorney General Jeff Sessions said the memo on marijuana enforcement he issued yesterday represented "a return to the rule of law." White House Press Secretary Sarah Sanders offered the same spin, telling reporters "the president believes in enforcing federal law…regardless of what the topic is, whether it's marijuana or whether it's immigration." But the the question for U.S. attorneys confronted by state-licensed marijuana suppliers was never whether they would enforce federal law; it was how they would enforce federal law.

National Review's David French agrees that Sessions' action amounts to "a restoration of the rule of law and the end of yet another unconstitutional Obama policy that privileged executive power over the American constitutional structure." He argues that the Obama administration tried to achieve through executive action what only Congress can do: repeal the federal ban on marijuana (a move that French supports). French is surely right that the Obama administration's prosecutorial restraint, as a solution to the conflict between state and federal marijuana laws, was vastly inferior to legislation making the federal ban inapplicable to people who comply with state law. But even if U.S. attorneys use their discretion differently in response to Sessions' memo (and it's not clear they will), they cannot avoid picking and choosing among cases, because it is impossible to "enforce federal law" against all violators, or even a meaningful share of them.

Marijuana enforcement is primarily a state responsibility, with the feds accounting for less than 1 percent of arrests. U.S. attorneys have never prosecuted more than a minuscule percentage of federal drug law violations. They have always had to decide which drug cases were worth pursuing, and they have always had very broad discretion in doing so, for better or worse. Those decisions became more complicated as more and more states opted out of marijuana prohibition, because the Justice Department could no longer count on state and local help in enforcing the federal ban. The DOJ never had the resources to enforce marijuana prohibition on its own, and now it has to be even pickier in selecting its targets.

The Obama administration's approach to this issue was a study in ambiguity. It could not simply announce that state-licensed marijuana growers and distributors, who openly commit federal felonies every day, would not be prosecuted as long as they complied with state law. Instead the guidance that Deputy Attorney General James Cole gave in 2013, which Sessions rescinded yesterday along with four related memos, said compliance with state law was one factor to consider in choosing marijuana cases. His reasoning was that oversight by "a strong and effective state regulatory system" makes it less likely that a marijuana supplier's activities will implicate "federal enforcement priorities" such as preventing violence, sales to minors, interstate smuggling, and "adverse public health consequences."

Cole did not tell U.S. attorneys to leave state-legal marijuana businesses alone. "The existence of a strong and effective state regulatory system, and an operation's compliance with such a system, may allay the threat that an operation's size poses to federal enforcement interests," he said. "Accordingly, in exercising prosecutorial discretion, prosecutors should not consider the size or commercial nature of a marijuana operation alone as a proxy for assessing whether marijuana trafficking implicates the Department's enforcement priorities listed above. Rather, prosecutors should continue to review marijuana cases on a case-by-case basis and weigh all available information and evidence, including, but not limited to, whether the operation is demonstrably in compliance with a strong and effective state regulatory system."

Contrary to what a Justice Department official claimed yesterday, that memo did not create a "safe harbor." Cole issued no orders to ignore federal law, made no promises, and gave no guarantees. In practice, however, U.S. attorneys since 2013 generally have refrained from prosecuting state-licensed cannabusinesses unless they violate state as well as federal law.

It is not clear whether that will change now that Sessions has scrapped the Cole memo. Yesterday the U.S. attorney in Colorado, where marijuana has been legal for five years, issued a press release saying Sessions' memo would not affect his prosecutorial practices. "The United States Attorney's Office in Colorado has already been guided by these principles in marijuana prosecutions—focusing in particular on identifying and prosecuting those who create the greatest safety threats to our communities around the state," said Bob Troyer, whom Sessions picked as interm U.S. attorney in November. "We will, consistent with the Attorney General's latest guidance, continue to take this approach in all of our work with our law enforcement partners throughout Colorado."

The U.S. attorney for the Southern District of California, which includes San Diego, welcomed Sessions' memo but likewise indicated that it would not change his approach to marijuana cases in a state where medical use has been allowed since 1996 and legal recreational sales began on Monday. "The Department of Justice is committed to reducing violent crime and enforcing the laws as enacted by Congress," said Adam Braverman, who like Troyer was appointed interim U.S. attorney by Sessions in November. "The cultivation, distribution, and possession of marijuana has long been and remains a violation of federal law. We will continue to utilize long-established prosecutorial priorities to carry out our mission to combat violent crime, disrupt and dismantle transnational criminal organizations, and stem the rising tide of the drug crisis."

With or without the Cole memo, Troyer and Braverman are being highly selective in deciding which federal drug felons are worth prosecuting. Troyer is focusing on "those who create the greatest safety threats to our communities," while Braverman is pursuing "our mission to combat violent crime, disrupt and dismantle transnational criminal organizations, and stem the rising tide of the drug crisis." Their descriptions of their prosecutorial priorities sound very much like the Cole memo. It is hard to see how the rule of law is any stronger now that Sessions has told Troyer and Braverman to continue exercising their vast discretion as they see fit.

If there is a rule-of-law problem here, it is similar to the one created by alcohol prohibition. The federal government has decided to ban peaceful activities that violate no one's rights, turning millions of otherwise law-abiding people across the country into criminals. The number of offenders is so large that the feds cannot hope to catch and punish a significant percentage of them, even with the cooperation of the states. Almost everyone who violates the law does so with impunity, while the high prevalence of these so-called crimes gives police and prosecutors dangerously broad authority to harass people and deprive them of their freedom. The treatment of the tiny share of offenders who happen to be arrested and prosecuted seems utterly arbitrary and unjust, inviting jury nullification.

The ban on marijuana is even more offensive to the rule of law than alcohol prohibition was, because it was never authorized by a constitutional amendment. The grotesque stretching of the Commerce Clause required to justify a law that applies to every trace of cannabis in America, whether or not it crosses state lines, down to the plant in a cancer patient's closet or the bag of buds in her dresser, is surely a bigger challenge to the rule of law than a weaselly memo suggesting how federal prosecutors should exercise a power they never should have been given.