The Afghan conflict is now officially America's longest war, surpassing Vietnam.

But another US-led war has been going on for a lot longer with even less to show for it and no exit in sight.

The War on Drugs, launched by President Nixon in 1971, makes the Afghan campaign seem like a blitzkrieg akin to June 1967 when Israel cleaned up Egypt, Syria and Jordan in six days and rested on the seventh.

Now some of those in the front line are airing their doubts. The questions are tentatively framed but the message is clear: We're fighting a losing battle.

As reported in last weekend's Herald, the ongoing carnage and erosion of central authority in parts of Latin America, notably Mexico, are prompting calls for the legalisation of drugs.

Now even Mexican President Felipe Calderon is calling for a debate. In 2006 he unleashed his army on the drug cartels; 28,000 deaths later the hardliner is having second thoughts.

Last month the chairman of the England and Wales Bar Council declared it was "rational" to consider decriminalising personal drug use.

This week the retiring president of the Royal College of Physicians called for laws to be reconsidered with a view to decriminalisation. This, he said, "could drastically reduce crime and improve health".

To say these remarks met with a kneejerk response is clichéd but appropriate since various British politicians reacted as reflexively as the lower leg does to a sharp tap on the patella.

Keith Vaz, chairman of the House of Commons Home Affairs committee, claimed to be "shocked" by the very suggestion of decriminalisation, which doesn't say much for the openness of his mind or the extent of his research.

A former member of the committee said the comments "were not entirely a helpful contribution to the debate". Well, that's the thing about debates: they're a clash of ideas. It's hard to have a decent debate if one side of the argument wants to dictate what the other side can say.

James Clappison MP went on: "There seems to be a very strong link between recreational drug use leading to addiction leading to crime fuelled by drug addiction."

What about the very strong link between recreational drug use, the black market, and the vast profits which make drug trafficking so lucrative that drug syndicates now control an estimated eight per cent of global GDP?

This insistence that decriminalisation is unthinkable would make more sense if prohibition was a resounding success. Perhaps that's setting the bar too high - let's say a partial success.

Still too high? How about something other than a largely futile, vastly wasteful, destructively counter-productive wallow in wilful ignorance which future generations will look back on in slack-jawed disbelief?

Take interdiction. Despite the best efforts of the sniffer dogs padding around airport terminals and the colossal human, financial and technical investment in stopping drugs getting to the market, supply keeps up with ever-growing demand. Since being invaded Afghanistan has regained its status as the world's leading producer of heroin and hashish which it lost under Taleban rule.

Seeing the presence of tens of thousands of western troops can't stop the flow, thermo-nuclear top-dressing would seem to be the only remaining option.

For opponents of decriminalisation it's an article of faith - and as such requires little elaboration - that no matter how bad the current situation, things would be much worse if drugs were legalised.

Leaving aside the fact that the production, distribution and sale of drugs would then be regulated and taxed, this view seems based on a very low opinion of homo sapiens: that we don't know what's good for us; that we can't resist temptation; that once it's legal we'll all jump at the opportunity to become drug-addled zombies; that actually we're not very sapient at all.

Vaz asserted that legalisation of drugs "would simply create the mistaken impression that these substances are not harmful."

The vast majority understand that these substances are or can be harmful and will avoid them whether they're legal or not. Most people who take drugs understand that they are or can be harmful but either don't care, or think it's a risk worth taking, or somehow persuade themselves that the nasty things that happen to others as a result of this habit won't happen to them.

Similar rationales are used by people who smoke or drink too much. Smoking has gone from cool to uncool, reflecting mounting public awareness of the harm it does and mounting public disapproval.

Smoking hasn't been eliminated, but it has declined. Drug use has gone the other way. Smoking is a dying a slow death. The only thing that can save it now is making it illegal.