A majority of Americans and Canadians believe that marijuana should be legal. The governments of the two countries, however, appear to be moving in very different directions.

Attorney General Jeff Sessions, who has been a staunch opponent of legalization for years, recently ordered a review of an Obama-era policy under which the federal government agreed not to interfere with state laws on marijuana, as long as the states took steps to regulate its distribution and use. Mr. Sessions’s apparent goal is to make Washington the ultimate authority.

Prime Minister Justin Trudeau of Canada, by contrast, would decentralize authority. His government on Thursday introduced legislation that would legalize the drug nationally by July 2018, and give the country’s provinces the power to regulate it.

Many nations have decriminalized marijuana, have allowed it to be prescribed for medical purposes or have effectively stopped enforcing laws against it. Should Mr. Trudeau’s bill pass, as expected, Canada would become only the second nation, after Uruguay, to completely legalize marijuana as a consumer product.

More than half of Americans now live in states that have legalized use of marijuana for medical or recreational purposes. A recent Quinnipiac University poll found that 59 percent of voters nationally believe that the drug should be legal and 71 percent say that the federal government should not enforce federal marijuana laws in states that legalized it. A recent Canadian poll found that support for legalization is even greater in that country, at 68 percent.

Given the robust level of public support in the United States, as well as a growing body of evidence that cannabis is less harmful than previously thought, Mr. Sessions’s decision to reopen the door to federal enforcement in states that have legalized the drug is wrongheaded. It is also potentially dangerous, because it could disrupt efforts by states and the District of Columbia to regulate marijuana, as well as help to re-energize the black market for the drug — and the crime and violence that go with it.

During the campaign, President Trump said he supported medical marijuana. And while he criticized its recreational use, he said during an October rally that legalization “should be a state issue.” A few days later, on election night, four states (California, Massachusetts, Maine and Nevada) voted to legalize recreational marijuana use, and four others (Arkansas, Florida, North Dakota and Montana) voted to permit medical use.

Mr. Sessions does not appear to have been paying attention to the states or, for that matter, to the president.

Mr. Trudeau came out in favor of full legalization in 2013 when he was in the opposition. After his party won a parliamentary majority in 2015, he created a task force to study the issue and make recommendations. (Canada legalized medical use of the drug in 2001.) The new bill would set a minimum age of 18 for buyers, though provincial governments would be able to set higher ages. People would be allowed to grow up to four plants at home, and provinces would regulate commercial growers and retail sales, including how the drug was priced and sold.

Before revoking state authority, Mr. Sessions should spend some time studying how the states that have legalized marijuana have been doing and listening to their political leaders. Gov. Jay Inslee of Washington, which legalized marijuana in a ballot initiative in 2012, says the attorney general has not responded to requests for meetings on the issue that he and the governors of Alaska, Colorado and Oregon have asked for.

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A version of this editorial appears in print on April 16, 2017, on Page SR10 of the New York edition with the headline: Canada Moves Boldly on Marijuana. Today’s Paper|Subscribe

https://www.nytimes.com/2017/04/15/opinion/sunday/canada-but-not-jeff-sessions-moves-boldly-on-marijuana.html