Attorney General Jeff Sessions has launched a new war on marijuana legalization.

On Thursday, Sessions rescinded guidances from former President Barack Obama’s administration that allowed states to legalize marijuana with minimal federal interference. In a statement, Sessions said that the move will allow federal prosecutors “to use previously established prosecutorial principles that provide them all the necessary tools to disrupt criminal organizations, tackle the growing drug crisis, and thwart violent crime across our country.”

In effect, this will let federal prosecutors use their own discretion to crack down on marijuana businesses in states where pot is legal for recreational purposes.

The move is a big deal for legalization efforts. While marijuana has been legalized for recreational purposes in eight states and Washington, DC, it remains illegal at the federal level — classified as a schedule 1 drug (the strictest such category in the scheduling system) with criminal penalties attached.

The Obama administration took a soft approach to the drug, essentially letting states legalize as long as they met certain criteria. But Sessions, who now heads the US Department of Justice, has pulled back the memos at the core of past lax enforcement.

The move could lead to a shift back to the days before the memos, when marijuana businesses that were deemed legal at the state level were often raided by federal law enforcement. That will cause more uncertainty in an industry that’s expected to grow by tens of billions of dollars in the next decade, while signaling to voters and officials that legalization at the state level is no longer enough for drug policy reform.

Advocates argue that legalization reduces racially skewed marijuana arrests and eliminates a black market for cannabis that helps fund criminal activity around the world, moving the pot trade from drug cartels to legal businesses that can be better regulated. Opponents say legalization will lead to more marijuana use, including among children and teens, causing all sorts of public health problems down the line.

The jury is still out on how this will all shake out. Although there’s evidence that laxer access does lead to more use, adolescent marijuana use has actually fallen in Colorado, which in 2014 became the first state to allow legal pot sales. And while marijuana use is linked to some health issues, pot is also nowhere near as dangerous as legal drugs like alcohol or tobacco — so even increased use may not cause huge problems.

That’s one reason what Colorado and other states are doing is valuable: It could give answers to these questions.

Sessions, however, is potentially moving to cut those experiments short.

To understand all that, though, let’s unpack exactly what Sessions is doing here.

How Sessions is cracking down on legal marijuana

Eight states and Washington, DC, have legalized marijuana for recreational purposes since 2012, when Colorado and Washington state became the first two to do so. The eight include California, the most populous state in the country and the latest, since January 1, to allow legal pot sales.

But even as these states allow cannabis for recreational use, pot remains illegal under federal law.

The Obama administration chose to enforce this law as leniently as possible in states where voters elected to legalize marijuana. Through a 2013 memo written by then–Deputy Attorney General James Cole (known as the Cole memo), it told the states that as long as they followed some rules (like not letting legal pot fall into kids’ hands or flow across state borders), the feds wouldn’t crack down. This let states carry out their legalization schemes with little federal interference — although federal law does still make it so legal pot businesses can’t claim certain tax deductions and easily access banking.

Some legalization advocates worried that Sessions, a vocal critic of legalization, would simply take a tougher interpretation of the memo — by, say, telling prosecutors to crack down on states that let any marijuana land in the hands of minors or across state lines (both of which are, to some extent, unavoidable no matter how strict a state is).

But Sessions has gone even further, ending the Cole memo and related guidances altogether. Since marijuana is illegal at the federal level, the change will let federal prosecutors go after state-legal marijuana at their own discretion — a return to the pre-memo days.

Before the memos were in place, it wasn’t rare for federal law enforcement to use their discretion to go after marijuana businesses that were doing nothing illegal at the state level — leading to high-profile raids during President George W. Bush’s administration and the early days of the Obama administration.

The concern for legalization advocates is Sessions’s policy change will lead to a chilling effect both among marijuana businesses, costing potential jobs and tax revenue, and efforts to legalize, hurting a movement that has picked up steam in recent years. It could even empower the Justice Department to shut down the entire legal marijuana industry, though that would take a lot of resources.

“This is going to create chaos in the dozens of states whose voters have chosen to regulate medical and adult use marijuana rather than leaving it in the hands of criminals,” Neill Franklin, executive director of the pro-legalization Law Enforcement Action Partnership, said in a statement. “If enforcement of laws are subject to the whims of individual prosecutors, no one will have any idea what is legal or what isn’t — because it could change from day to day.”

Indeed, anti-legalization advocates are celebrating the move because of the potential chilling effect. Smart Approaches to Marijuana (SAM), a leading anti-legalization group, said in a statement that Sessions’s change “makes investing in the marijuana industry a risky move.”

“This is a good day for public health. The days of safe harbor for multi-million dollar pot investments are over,” Kevin Sabet, head of SAM, said in a statement. “DOJ’s move will slow down the rise of Big Marijuana and stop the massive infusion of money going to fund pot candies, cookies, ice creams, and other kid-friendly pot edibles. Investor, banker, funder beware.”

Still, a lot of uncertainty remains. Justice Department officials couldn’t tell HuffPost whether Sessions’s move will actually lead to more anti-marijuana prosecutions. That even Justice Department officials don’t know the answers to such a basic question suggests there may be a lot of chaos around this decision. “And that might be what [the Justice Department] intended,” Ryan Reilly of HuffPost tweeted.

Sessions has long been an opponent of marijuana legalization

The move, however, isn’t very surprising for Sessions, who once said that “good people don’t smoke marijuana” and who has long been an opponent of legalizing marijuana for any purpose.

“We need grown-ups in charge in Washington to say marijuana is not the kind of thing that ought to be legalized, it ought not to be minimized, that it’s in fact a very real danger,” he said as a senator in 2016.

In July 2017, Sessions sent a letter to Washington state officials that revealed his skepticism of legalization. “Congress has determined that marijuana is a dangerous drug and that the illegal distribution and sale of marijuana is a crime,” he wrote. “The Department remains committed to enforcing the Controlled Substances Act in a manner that efficiently applies our resources to address the most significant threats to public health and safety.”

Sessions cited a 2016 report by the Northwest High Intensity Drug Trafficking Area, claiming that it “raises serious questions about the efficacy of marijuana ‘regulatory structures’ in your state” because, among other issues, pot remains loosely regulated and legal pot from Washington state “has been found to have been destined for 43 different states.”

As Christopher Ingraham pointed out at the Washington Post, these types of federal reports are notoriously questionable. They tend to cherry-pick data, presenting a case favorable to the war on drugs and prohibition. The data from the 2016 report is also limited, covering only the first several months of Washington state allowing pot sales.

But Sessions has appeared to rely on these kinds of figures to shape his worldview — one that is highly critical of legalization.

One big question is whether Sessions’s boss, President Donald Trump, will welcome this new anti-pot crackdown. On the campaign trail, Trump said that he would like to leave marijuana legalization to the states. This is a typical Republican stance — that states should generally be allowed to experiment with policies while the federal government gets out of the way.

But after Trump was elected, his press secretary told reporters, “There’s still a federal law that we need to abide by when it comes to recreational marijuana and other drugs of that nature.” And the Justice Department, in rescinding the Obama-era memos, argued that “the rule of law” must be followed — and since marijuana is illegal, the Justice Department has an obligation to enforce that law.

With Sessions’s move, it seems concerns about the rule of law ultimately won over desires to let federalism and state innovation take root.

This tension is likely as long as federal and state laws are in conflict

The core issue here is the conflict between federal and state laws on marijuana.

Over the past couple of decades, states have moved to legalize marijuana for medical and recreational purposes. But federal law has remained largely unchanged, keeping cannabis illegal at the federal level for any use — medical or recreational.

This makes it possible for federal marijuana policy to change dramatically, based on how harshly the current administration wants to enforce federal law. We’ve seen this shift multiple times now: The Bush administration cracked down on legal marijuana, then the Obama administration over time relaxed federal enforcement, and now the Trump administration, with Sessions at the helm, is potentially ramping up enforcement again.

Congress could do something to change this. It could legalize marijuana at the federal level, leaving it to the states to decide what to do about marijuana.

Or it could at least limit federal enforcement. This is what Congress did with medical marijuana, passing a budget rider that prohibits the Justice Department from using federal funds to crack down on medical marijuana establishments and users in states where cannabis is legal for such purposes. That forced federal law enforcement to pull back on states that legalized medical pot (which is why Thursday’s move hasn’t drawn as much concern for medical marijuana states), although the Justice Department has tried to argue that the budget rider doesn’t block some medical marijuana–related prosecutions.

But since marijuana remains illegal and no such rider exists for recreational pot, the Trump administration — or any other administration — can move to enforce federal anti-marijuana laws as it deems fit. (That also means a new administration, a new attorney general, or even Sessions himself could reverse the current policy.)

It is bizarre that Congress has not acted yet. Does Congress really want to let state ballot initiatives created by advocacy groups decide the future of a major drug policy issue? Is Congress really okay with the creation of another for-profit industry focused on marketing and selling a recreational drug, given the track record of the alcohol and tobacco industries? Do federal lawmakers really not want to make sure there’s at least some regulatory floor for how marijuana is grown, transported, advertised, and sold? Is Congress really going to do nothing as a giant new industry takes root in America?

One major hurdle for Congress, though, may be international treaties.

From the 1960s through the 1980s, much of the world, including the US, signed on to three major international drug policy treaties: the Single Convention on Narcotic Drugs of 1961, the Convention on Psychotropic Drugs of 1971, and the United Nations Convention Against Illicit Traffic in Narcotic Drugs and Psychotropic Substances of 1988. Combined, the treaties require participants to limit and even prohibit the possession, use, trade, and distribution of drugs outside of medical and scientific purposes, and they must work together to stop international drug trafficking.

There is some debate about whether these treaties stop countries from decriminalizing marijuana — when criminal penalties are repealed but civil ones remain in place — and legalizing medical marijuana. But one thing the treaties are absolutely clear on is that illicit drugs aren’t to be allowed for recreational use and certainly not for recreational sales.

There are ways around this. Bolivia, for example, effectively withdrew from the 1961 Single Convention on Narcotic Drugs; then it rejoined with a “reservation” allowing the use of coca leaves within its own borders. The move could have been blocked by one-third of the parties to the treaty — which would amount to more than 60 nations — but only 15 joined in opposition.

The US could do something similar with marijuana. Or it could try to ignore the treaties (as Uruguay has essentially done) or find another solution altogether. (So far, the US has by and large avoided these issues with state-level legalization because marijuana remains illegal at the federal level. But that could change if Congress takes action.)

Congress has mostly dodged these questions to this point. But as more states legalize marijuana, and as officials like Sessions follow their own views on marijuana policy, the conflict between federal and state laws will likely become a bigger issue.

Sessions’s move could lead to a backlash

The risk for Sessions and the Trump administration is their new war on legal marijuana could prompt a backlash.

For one, Sessions’s policy lets federal law enforcement go against the will of the voters. So far, the eight states that have legalized marijuana have done so through ballot initiatives with voter support. The federal government is effectively rejecting those votes by going after legal pot in those states — and voters could take offense to that.

More broadly, marijuana legalization is fairly popular at the national level. Gallup’s latest survey in 2017 found that 64 percent of US adults back legalization, up from 36 percent more than a decade before. Gallup even found that a majority of Republicans now support legalization. (One caveat: Anti-legalization advocates argue that if surveys offered options between decriminalization, medical legalization, and recreational legalization, voters would be much less likely to say that they back full legalization.)

So the attorney general stands in contrast to public opinion, including that of a majority of Republicans.

Democrats in Congress, such as House Minority Leader Nancy Pelosi and Sen. Cory Booker (NJ), have pushed back against the Obama-era memos’ rollback. Some Republicans have as well; Sen. Cory Gardner (R-CO), for one, threatened to hold up Justice Department nominees in the Senate in response to Sessions’s decision.

But Sessions, long a believer in punitive anti-drug and “tough on crime” policies, has moved forward anyway — launching a new war on legal marijuana.