Smoking marijuana is going to be legal in the United States. Maybe not tomorrow but definitely in Colorado and Washington come January, and as the movement builds, it's only a matter of time before the rest of the nation follows suit.

The federal government knows legalization in two states will have an effect nationally, but for the first time, Americans don't seem to mind. In October, Gallup reported that a majority — 58 percent — of Americans surveyed favor legalization, a dramatic 27 percentage-point increase since 2000 and 9 times the increase in the preceding 23 years combined. It looks more or less like the trend in support for gay marriage. By 2020, I'm optimistic we'll have seen the last arrest for marijuana use in the United States, bringing to an end a 50-year mistake that never had to be made in the first place.

The year was 1970, and Congress had just passed and President Richard Nixon just signed the Controlled Substances Act, in what would become the first shot of a long and bloody drug war. The act created the five-level schedule of controlled substances, but it didn't decide which drugs would go where right away. Instead, it called into being the National Commission on Marihuana and Drug Abuse, which went about developing a policy.

What the commission came back with in 1972 sounds uncanny to anyone who grew up in America in the last 40 years. Its report, "Marihuana, a Signal of Misunderstanding," didn't advocate a different approach to prohibition, but instead mused on the limits of human understanding, the rise of leisure time and the future of surveillance. The report is modest and sympathetic to everyone involved, with an acute focus on the consequences of its recommendations. In short, it's everything U.S. marijuana policy hasn't been since.

With sections like The Need for Perspective, Drugs in a Free Society and Marihuana and the Dominant Social Order, "A Signal of Misunderstanding" is the cool uncle of government reports. After a thorough review, the commission decided that marijuana was mostly a symbolic problem rather than a real public-health crisis. The proper role of the federal government, it wrote, was to lower the emotional stakes of the debate so that they match the drug's actual risks. The commission concluded that the criminal-justice system was the wrong way to address personal use. "The existing social and legal policy (i.e., prohibition) is out of proportion to the individual and social harm engendered by the use of the drug," a group of adults reasonably concluded. Yes, more than 40 years ago, a government commission headed by a Republican, reporting to Nixon, tasked with developing what would become marijuana policy for much of the world, told the feds to chill out. The whole story sounds like stoner apocrypha, but it's true.

The report was not, however, to become policy. Nixon wasn't about to extend understanding to the yippie radicals who were the face of marijuana use as well as his extraparliamentary opposition, so the Department of Health's temporary — and wholly arbitrary — classification of weed as a Schedule I narcotic would stand.

Since then, marijuana prohibition has been the kind of unmitigated disaster that makes America's ill-fated dalliance with alcohol prohibition look like responsible lawmaking in comparison. Law professor Michelle Alexander went so far as to call the drug war — and in particular the war on black marijuana users and their communities — "the new Jim Crow" in her hit book of the same title. Drug arrests, she writes, increased over 1,000 percent in the 1990s, and 80 percent of that increase was for marijuana possession. She describes in detail a system of incentives in which local law-enforcement agencies were seduced by federal anti-drug money and changed their policing practices to meet crude progress metrics. It's not the American people who got addicted to weed — it's their cops.