When Gordon Brown flies to Washington on Sunday, top of the summit agenda will be the discussion of more punitive sanctions on Iran for its refusal to stop enriching uranium. It’s perverse. It’s wrong. But it’s a fair bet that Brown and Bush will come out of their talks firm in purpose, serious in tone, in tightening the squeeze on America’s No. 1 enemy of choice.

Why the obsession with Iran? For the US it is simple. Look at virtually every problem America now has in the Middle East, examine all the hopes and plans it had for the region when it first marched into Iraq, and you find Iran as the single and most powerful obstacle to its success. Its alleged nuclear ambitions (and they are just alleged at the moment) threaten Israel and the balance of power in the region in a way even Saddam Hussein never did. Its arms and training are said to be behind the insurgency in Iraq and even the Taleban attacks in Afghanistan.

For a president whose Iraq policy has lost the support of the American public and whose personal rating has fallen to an all-time low, Iran is not just a useful scapegoat but an enemy whose defeat might yet restore his glory. Which is why many US politicians and commentators still believe that, despite the opposition of Congress, military advice and public disapproval, the White House could yet follow Vice President Dick Cheney’s advice and go for a military strike against Iran’s nuclear facilities.

That in turn could explain why the Europeans remain so inflexible in their negotiations with Iran and so eager to pursue the course of increasing sanctions on the regime. The more fearful the Europeans are that Washington may yet resort to arms (and no one in Whitehall is confident they won’t), the more the so-called E3 of the UK, Germany and France seem ready to up the ante of alternative action. Iran has few friends in Paris or Berlin in any case, even less now that the more activist Nicolas Sarkozy is French president. Having opposed the war, the two continental capitals are keen to find common ground with Washington. Iran can be presented as precisely the kind of threat they can all unite against.

Just because it is in everyone’s interest at the moment to scapegoat Iran, however, doesn’t make it right. Even supposing for a moment that Iran really was a major source of instability, sanctions would still not be the answer.

The trouble with sanctions is that, far from undermining or squeezing regimes, they tend to do the opposite. By isolating a country you increase the paranoia and power of the ruling government, which can appeal for popular support against foreign pressure and introduce tighter internal security in response. The sanctions against Iraq in the decade leading up to the invasion are the classic example. But it is equally true of Zimbabwe, Burma and Cuba. South Africa was the exception. There you had a ruling white minority who could be discomforted by international isolation. But Iran? Sanctions, including the sanctions on financial transfers and investment, can certainly hurt individuals, and probably do. But the people they don’t hurt are President Ahmadinejad and his conservative religious supporters, who find their position strengthened by foreign threats and who are largely immune to them.

The Iranian regime is not a nice one. In many ways it is deeply oppressive, and getting worse. But the government in Tehran is not monolithic, nor is its president all-powerful. Iran may be — although there is no proof as yet — the source of weaponry and even trained fighters crossing into Iraq and Afghanistan. And powerful elements in its ruling elite may want at least the technological possibility of nuclear weaponry, if not the weaponry itself.

But it is also true that Iran has a perfectly good case for pursuing uranium enrichment, as it is allowed under the Nuclear Proliferation Treaty; that it is a signatory to the NPT and has not yet withdrawn from it; and that it is a regional power with strong interests in the future of Iraq and Afghanistan, whatever the precise nature of its support for groups there. Given more encouraging signals from Washington and the EU, there is no reason to believe that Tehran would not respond.

The trouble is that it is not getting those signals. Just the opposite. The whole tone of statements from London and Washington has been one of demands on Iran to do this, to prove that, to forego its interests in the region. We are locked into a classic struggle of pride and macho confrontation that is leading nowhere,

If Gordon Brown really wants to indicate a new and independent foreign policy when he meets President Bush, it is on Iran far more than Iraq that he should set out his stall.