The next 25 years took the nation from Bill Clinton, who famously "didn't inhale," to Barack Obama, who most emphatically did.

Now, in just a few short years, public opinion has moved so dramatically toward general acceptance that even those who champion legalization are surprised at how quickly attitudes are changing and states are moving to approve the drug—for medical use and just for fun.

It is a moment in America that is rife with contradictions:

—People are looking more kindly on marijuana even as science reveals more about the drug's potential dangers, particularly for young people.

—States are giving the green light to the drug in direct defiance of a federal prohibition on its use.

—Exploration of the potential medical benefit is limited by high federal hurdles to research.

Washington policymakers seem reluctant to deal with any of it.

Richard Bonnie, a University of Virginia law professor who worked for a national commission that recommended decriminalizing marijuana in 1972, sees the public taking a big leap from prohibition to a more laissez-faire approach without full deliberation.

"It's a remarkable story historically," he says. "But as a matter of public policy, it's a little worrisome."