This is incredibly good news:

The United States government took an historic step back from its long-running drug war on Thursday, when Attorney General Eric Holder informed the governors of Washington and Colorado that the Department of Justice would allow the states to create a regime that would regulate and implement the ballot initiatives that legalized the use of marijuana for adults.

A Justice Department official said that Holder told the governors in a joint phone call early Thursday afternoon that the department would take a "trust but verify approach" to the state laws. DOJ is reserving its right to file a preemption lawsuit at a later date, since the states' regulation of marijuana is illegal under the Controlled Substances Act.

Washington State and Colorado can now apparently proceed with licensing growers; those growers can cultivate their farms, they can collaborate with distribution companies just like beer, and soon stores will open where adults can go purchase a bag of pot.

This is really happening.

We can be pretty confident this would never happen under a Romney Administration. But I wouldn't count on this being a carte blanche from the Obama administration, either. They lay out guidelines, much like those for medical marijuana, that say federal prosecutors will still target individuals who step outside the bounds of their own state laws. Here's more:

Deputy Attorney General James Cole also issued a three-and-a-half page memo to U.S. attorneys across the country on Thursday outlining eight priorities for federal prosecutors enforcing marijuana laws. According to the guidance, DOJ will still prosecute individuals or entities to prevent:

• the distribution of marijuana to minors;
• revenue from the sale of marijuana from going to criminal enterprises, gangs and cartels;
• the diversion of marijuana from states where it is legal under state law in some form to other states;
• state-authorized marijuana activity from being used as a cover or pretext for the trafficking of other illegal drugs or other illegal activity;
• violence and the use of firearms in the cultivation and distribution of marijuana
• drugged driving and the exacerbation of other adverse public health consequences associated with marijuana use;
• growing of marijuana on public lands and the attendant public safety and environmental dangers posed by marijuana production on public lands;
• preventing marijuana possession or use on federal property.

If Washington's and Colorado's legal weed starts showing up in California, if kids start buying pot at the stores in significant numbers, or people with licenses are running shady profit deals outside the regulatory scheme—especially out of state—you can be sure the DOJ will seek an injunction against the states to shut this down. They still may not win, but they will have grounds to try. For now, however, this not only fully legalizes pot in two states... this proclaims that California and any other state that can muster a ballot measure has the green light to legalize pot, too.

This is also proof that legalizing pot—just like gay marriage and a zillion other issues—begins at the state level, not in Congress.

In the end, Obama had no political ground to gain in seeking an injunction—if anything, he wants ineffective drug-control policy to be dismantled on his watch as a means toward racial justice and economic stability. But as I wrote in Slate a few months ago, Obama also needed to butt out for tactical reasons. The supposed goals of banning marijuana are making streets safer, weakening gangs, and keeping kids off pot. Obama knows full well that legalizing and regulating the pot market is likely to succeed far better at those three goals banning it has.