The Trump administration likes to say that policy is best set by the states and not in Washington.

Except when it comes to marijuana policy. There, the states don’t know what they’re doing.

So it was no surprise last week when Attorney General Jeff Sessions sent a message to federal prosecutors instructing them to ignore Obama-era memos that discouraged prosecution of activities if they are legal under state law.

In states like Maine that allow medical use and even recreational use, businesses could grow and distribute marijuana as long as they met certain conditions, like keeping the drug away from minors, keeping it out of states that haven’t liberalized their laws and keeping profits away from criminal enterprises.

That regime let people make legal business investments and gave the states the prospect of collecting tax revenue from sales that would otherwise be made on the illegal black market.

Sessions’ memo doesn’t promise a renewed war on marijuana, but it does create enough uncertainty for legal businesses and state regulators to set back efforts to establish a market on the right side of the law. That’s good news for anti-pot hardliners, but it’s also good news for illegal dealers who can continue to make unregulated, tax-free sales to people who can’t be discouraged by legal sanctions for possession or use in private.

Sessions’ memo doesn’t address the underlying problem with marijuana policy, which is that state and federal laws are in conflict. Although it’s legal for at least some uses in about half the states, it is always illegal under federal law. Given the chance, voters tend to approve legalization, but not every state lets citizens approve laws directly, and there is no federal referendum policy.

President Obama’s approach was no solution, either. But at least it recognized that state laws have been changed through a democratic process.

While imperfect, it pointed the nation in a direction that Sessions’ memo giving discretion to local prosecutors doesn’t really change. U.S. attorneys are residents of the states that they serve in, and are at least aware of local political sentiments, even if they are not directly elected.

It’s possible that prosecutors, like Maine’s U.S. Attorney Halsey Frank, will choose to set higher priority for federal crimes where there is no state-law conflict, like cross state commerce in opioids, illegal guns and human trafficking.

And there is some benefit that comes from having the federal government step back and watch what happens when marijuana is legal in some states and not in others. There were many questions raised in the 2016 referendum campaign that have not been answered. This is an opportunity to find the truth.

Does legalized adult use of marijuana result in more use by minors, as opponents claimed?

Would state-sanctioned sales mean less profit for illegal drug dealers?

Is marijuana use really less harmful than alcohol, as legalization proponents claim? Or would the combination of legal pot and legal booze mean there are twice as many impaired people at work, school and driving on our roads?

Letting this process develop without interference from Washington would at least provide hard evidence about what’s really at stake. If some Mainers don’t like the idea of living under a microscope, they should consider the 40-year experiment that we just lived through.

In 1980, there were about 15,000 people behind bars for drug dealing. Today there are 450,000 people in jails and prisons for the same offenses. “And the prices of all major drugs are down dramatically,” said Mark Kleiman, a criminologist who specializes in drug policy. “So if the question is do longer sentences lead to a higher drug price and therefore less drug consumption, the answer is no.”