The trend to legalize marijuana in a growing number of states across the country – as medical treatment and more recently simply to get high – inevitably created a conflict with federal laws listing cannabis as an illegal drug, Washington claiming that its prohibition trumped state authority regarding pot.

On Thursday, Uncle Sam blinked, sort of.

In a memo to US attorneys in the 50 states, Deputy Attorney General James Cole in effect says Washington will leave it to states to regulate individual marijuana use, recreationally or for medicinal purposes. As long as states “implement strong and effective regulatory and enforcement systems,” the federal government will not go after pot use deemed legal under state law, the memo declares.

So far, 18 states and the District of Columbia have legalized medical marijuana. In last year’s elections, voters in Colorado and Washington State approved recreational pot use, and Thursday’s announcement could set the stage for more states to legalize marijuana. Alaska is scheduled to vote on the question next year, and a few other states plan similar votes in 2016.

That seems to be a logical trend given the movement in public opinion.

In April, the Pew Research Center for the first time found majority support (52-45 percent) for legalizing marijuana – a noticeable 11 percent increase in support just since 2010, and a whopping rise since 1969 when Gallup found that 84 percent of those surveyed opposed legalization. Some 48 percent of Americans say they have tried marijuana, up from 38 percent a decade ago.

The recent Pew poll also found that a larger number of Americans – 60 percent – agree that “the federal government should not enforce federal laws prohibiting the use of marijuana in states where it is legal.”

The Justice Department memo was not a big surprise. President Obama had signaled his position last December when he told Barbara Walters in an ABC News interview, “It would not make sense for us to see a top priority as going after recreational users in states that have determined that it’s legal.”

As with most things regarding the law and politics, the devil is in the details.

Officials in Colorado and Washington are working out the specifics of how to regulate the sale and personal use of marijuana. The new Colorado law allows municipalities to regulate retail sales of marijuana for recreational use. They can also decide to not allow such sales at all in their jurisdiction.

But the general enforcement attitude toward individuals possessing and using small amounts of pot seems relatively benign so far.

At Seattle’s recent annual Hempfest, police did not cite those smoking marijuana in public even though that remains technically illegal in Washington State. Instead, to those feeling the “munchies,” officers passed out bags of Doritos chips with friendly printed advice about following the law.

The Justice Department memo, on the other hand, is all business, reading more like a warning than political or legal capitulation.

“If state reinforcement efforts are not sufficiently robust … the federal government may seek to challenge the [state] regulatory scheme itself in addition to continuing to bring individual enforcement actions, including criminal prosecutions…,” it states.

Under the new guidelines, the federal government's top investigative priorities include preventing the distribution of marijuana to minors; preventing sales revenue from going to criminal enterprises, gangs and cartels; preventing state-authorized marijuana activity from being used as a cover or pretext for the trafficking of other illegal drugs or other illegal activity; preventing violence and the use of firearms in the cultivation and distribution of marijuana; preventing the growing of marijuana on public lands; and preventing the diversion of marijuana outside of states where it is legal under state law.

• The report includes material from the Associated Press.