Historians of the future will doubtless marvel that a great and powerful republic, founded in part on “liberty and the pursuit of happiness” but now suffering from difficult economic times would waste billions of dollars every year in a futile war against a humble plant.

That plant, of course, is hemp — source of oil, fiber and a mild psychoactive drug. It’s so mild that in all of history, no one has ever died from a marijuana overdose.

And those who used it in their youth, like the three most recent American presidents (Clinton claimed he “didn’t inhale,” Bush was “young and foolish” in his jejune days, and Obama confessed that “pot had helped” during his youth), somehow managed to go on to reasonably productive lives.

So why is the stuff still illegal?

For one thing, there’s an immense federal bureaucracy, the Drug Enforcement Administration, which naturally seeks to stay in business. As long as pot is illegal, the DEA has plenty of work. And when the need arises for a headline to show that the DEA is on the ball, its agents can always drive to some home that uses too much electricity, shoot the dogs, kick in the door, and announce that American youth are protected because it just seized plants with an estimated street value of $4.2 gazillion.

For another, there’s our pharmaceutical industry, a major source of campaign contributions. The pill-makers buy candidates so they can protect their revenue streams.

Now, it might be too much to expect the federal government to move sensibly here. There are, after all, two wars and a crumbling economy to contend with. But Colorado could help itself by legalizing the cultivation, sale and use of marijuana with a reasonable excise tax of $25 an ounce.

It would save money in several ways, like lower law-enforcement costs, as well as a reduction in the prison population. Further, the corruption and violence associated with black markets should diminish.

More money would circulate in our state, as Colorado hemp farmers received money now going to Mexican drug cartels. Profitable farms mean that open space gets preserved through market mechanisms, rather than taxes and zoning. Further, it might enhance tourism, at least until other states catch on.

One possible snag is the federal government. No matter how sensible we make our state laws, there would still be draconian and moronic federal laws enforced by federal agents.

So initially, the marijuana excise tax proceeds should go to our state attorney general’s office, with instructions that the money be used to defend all Coloradans charged with marijuana violations that are crimes under federal law but not under our enlightened state law.

In other words, every “probable cause” for a search warrant would be vigorously contested. The chain of evidence would come under intense scrutiny. The credibility of informants and agents would be subject to brutal cross-examination.

Every such trial — our tenacious defense teams would never plea-bargain — would be a grinding ordeal for the U.S. Attorney’s office. The federal Department of Justice would soon move its prosecutorial resources away from pot and toward real crimes that people care about.

The downside? Maybe a few more lazy potheads munching junk food. But in today’s economy, there aren’t jobs for them anyway, so where’s the harm to society?

Contrast that with the benefits of reduced spending on cops and prisons, a boost to Colorado agriculture, and increased revenue for our hard-pressed state government, if we’d just give up on this silly war against a plant.

Ed Quillen (ed@cozine.com) of Salida is a freelance writer and history buff, and a frequent contributor to The Post.