In a decision with historic implications, the federal government announced Thursday that it will not stand in the way of marijuana legalization in Colorado or Washington state.

But it will be watching closely.

Deputy U.S. Attorney General James Cole wrote in a memo sent Thursday to federal prosecutors that it will not be a priority to block landmark marijuana-legalization laws in the two states. The federal government also will not make it a priority to close down recreational marijuana stores, so long as the stores abide by state regulations, according to the memo.

However, if the two states are unable to keep pot away from kids, keep criminal gangs out of the marijuana industry or keep marijuana from being trafficked into neighboring states, among other concerns, Cole wrote that the federal government would consider cracking down.

“A system … must not only contain robust controls and procedures on paper; it must also be effective in practice,” Cole wrote in the memo.

President Barack Obama and federal officials have previously said marijuana users would not be targets for prosecution. But Thursday’s memo is the first time federal officials have ever offered state-legal marijuana businesses that follow strict rules explicit cover from prosecution. That means the first-of-their-kind — and federally illegal — recreational marijuana businesses scheduled to open in Colorado and Washington early next year will be able to operate at least initially without federal interference.

“Today’s announcement shows the federal government is respecting the will of Colorado voters,” Gov. John Hickenlooper said in a statement. “We share with the federal government its priorities going forward.”

Hickenlooper learned about the memo Thursday morning in a phone call with U.S. Attorney General Eric Holder and the governor of Washington.

Marijuana advocates generally hailed the memo as a substantial victory. Mason Tvert, one of the advocates behind Amendment 64, the measure that legalized pot in Colorado, said the memo is “a major and historic step toward ending marijuana prohibition.” In addition to offering guidance for Colorado and Washington, the memo also offers a blueprint for other states considering marijuana legalization, advocates said.

“It sends a clear signal that states are free to determine their own policies when it comes to marijuana,” Tvert said.

Anti-marijuana groups expressed alarm. Calvina Fay, the head of the group Save Our Society From Drugs, said the Justice Department “has surrendered to legalizing marijuana.”

“(Holder) is essentially setting up a tsunami that will no doubt result in far too many of America’s young people being subjected to chemical slavery,” Fay said in a statement.

Robert Mikos, a Vanderbilt law professor who has studied the federal government’s stance toward marijuana, said the memo signals a significant softening of its position on pot.

“This is a really big deal,” Mikos said. “It’s a major policy shift from the Department of Justice.”

But, as some marijuana-policy experts warned, the memo is also limited in its application.

The memo does not change federal law regarding marijuana. Cannabis will remain a Schedule I controlled substance — the most tightly regulated type of drug under federal law — and people who use, grow or sell marijuana remain at risk for prosecution. The memo does not provide any defense against marijuana charges in federal court.

Because pot is still illegal federally, banks will likely continue to refuse marijuana business accounts, employers can continue to fire workers who smoke pot off-the-job and marijuana users who receive federal aid or live in federal housing will remain at risk.

“This is big; it’s not everything,” said Sam Kamin, a University of Denver law professor who studies marijuana laws. “It doesn’t solve all the problems.”

The policy articulated in the memo can also be revised at any time, and it could be scrapped under a different attorney general.

“All bets are off as to what will happen in the next administration,” Mikos said.

Still, the memo represents nine months of work and contemplation by federal officials following Colorado and Washington’s legalization of marijuana last November. As lawmakers and regulators in both states craft rules for the recreational marijuana industry, the memo shows federal officials realized something has fundamentally changed in marijuana enforcement.

As Cole wrote in the memo, federal and state law enforcement agencies were once partners in pot policing — with the feds taking the big crimes and the states handling the small ones. Colorado’s and Washington’s new laws severed that relationship, and the federal government doesn’t have the muscle to pick up the slack.

“The Department is … committed to using its limited investigative and prosecutorial resources to address the most significant threats in the most effective, consistent and rational way,” Cole wrote.

As guidance to prosecutors, the memo lays out eight federal priorities when deciding whether to prosecute a marijuana case.

They are:

• Preventing marijuana distribution to minors;

• Preventing money from sales from going to criminal groups;

• Preventing the diversion of marijuana from states where it is legal to states where it is illegal;

• Preventing criminal groups from using state laws as cover for trafficking of other illegal drugs;

• Preventing violence and the use of illegal firearms;

• Preventing drugged driving and marijuana-related public health problems;

• Preventing the growing of marijuana on public lands;

• Preventing marijuana possession or use on federal property.

Contrary to previous DOJ guidance, the size or profitability of legal marijuana businesses can no longer be the only factor in assessing whether they should be a target for prosecution.

Mike Elliott, the executive director of the Denver-based Medical Marijuana Industry Group, said lawmakers and regulators in Colorado have already been hard at work addressing the Justice Department’s priorities. Lawmakers passed a bill on stoned driving last session. New rules for recreational marijuana stores are poised to require detailed inventory tracking and ban advertisements in places where kids can see them.

“We’ve all been working to comply with what we heard they care about,” Elliott said. “But a lot of it is common sense, too.”

John Ingold: 303-954-1068, jingold@denverpost.com or twitter.com/john_ingold