They don't give up. After a decade of blood-drenched failure in Afghanistan and Iraq, violent destabilisation of Pakistan and Yemen, the devastation of Lebanon and slaughter in Libya, you might hope the US and its friends had had their fill of invasion and intervention in the Muslim world.

It seems not. For months the evidence has been growing that a US-Israeli stealth war against Iran has already begun, backed by Britain and France. Covert support for armed opposition groups has spread into a campaign of assassinations of Iranian scientists, cyber warfare, attacks on military and missile installations, and the killing of an Iranian general, among others.

The attacks are not directly acknowledged, but accompanied by intelligence-steered nods and winks as the media are fed a stream of hostile tales – the most outlandish so far being an alleged Iranian plot to kill the Saudi ambassador to the US – and the western powers ratchet up pressure for yet more sanctions over Iran's nuclear programme.

The British government's decision to take the lead in imposing sanctions on all Iranian banks and pressing for an EU boycott of Iranian oil triggered the trashing of its embassy in Tehran by demonstrators last week and subsequent expulsion of Iranian diplomats from London.

It's a taste of how the conflict can quickly escalate, as was the downing of a US spyplane over Iranian territory at the weekend. What one Israeli official has called a "new kind of war" has the potential to become a much more old-fashioned one that would threaten us all.

Last month the Guardian was told by British defence ministry officials that if the US brought forward plans to attack Iran (as they believed it might), it would "seek, and receive, UK military help", including sea and air support and permission to use the ethnically cleansed British island colony of Diego Garcia.

Whether the officials' motive was to soften up public opinion for war or warn against it, this was an extraordinary admission: the Britain military establishment fully expects to take part in an unprovoked US attack on Iran – just as it did against Iraq eight years ago.

What was dismissed by the former foreign secretary Jack Straw as "unthinkable", and for David Cameron became an option not to be taken "off the table", now turns out to be as good as a done deal if the US decides to launch a war that no one can seriously doubt would have disastrous consequences. But there has been no debate in parliament and no mainstream political challenge to what Straw's successor, David Miliband, this week called the danger of "sleepwalking into a war with Iran". That's all the more shocking because the case against Iran is so spectacularly flimsy.

There is in fact no reliable evidence that Iran is engaged in a nuclear weapons programme. The latest International Atomic Energy Agency report once again failed to produce a smoking gun, despite the best efforts of its new director general, Yukiya Amano – described in a WikiLeaks cable as "solidly in the US court on every strategic decision".

As in the runup to the invasion of Iraq, the strongest allegations are based on "secret intelligence" from western governments. But even the US national intelligence director, James Clapper, has accepted that the evidence suggests Iran suspended any weapons programme in 2003 and has not reactivated it.

The whole campaign has an Alice in Wonderland quality about it. Iran, which says it doesn't want nuclear weapons, is surrounded by nuclear-weapon states: the US – which also has forces in neighbouring Afghanistan and Iraq, as well as military bases across the region – Israel, Russia, Pakistan and India.

Iran is of course an authoritarian state, though not as repressive as western allies such as Saudi Arabia. But it has invaded no one in 200 years. It was itself invaded by Iraq with western support in the 1980s, while the US and Israel have attacked 10 countries or territories between them in the past decade. Britain exploited, occupied and overthrew governments in Iran for over a century. So who threatens who exactly?

As Israel's defence minister, Ehud Barak, said recently, if he were an Iranian leader he would "probably" want nuclear weapons. Claims that Iran poses an "existential threat" to Israel because President Ahmadinejad said the state "must vanish from the page of time" bear no relation to reality. Even if Iran were to achieve a nuclear threshold, as some suspect is its real ambition, it would be in no position to attack a state with upwards of 300 nuclear warheads, backed to the hilt by the world's most powerful military force.

The real challenge posed by Iran to the US and Israel has been as an independent regional power, allied to Syria and the Lebanese Hezbollah and Palestinian Hamas movements. As US troops withdraw from Iraq, Saudi Arabia fans sectarianism, and Syrian opposition leaders promise a break with Iran, Hezbollah and Hamas, the threat of proxy wars is growing across the region.

A US or Israeli attack on Iran would turn that regional maelstrom into a global firestorm. Iran would certainly retaliate directly and through allies against Israel, the US and US Gulf client states, and block the 20% of global oil supplies shipped through the Strait of Hormuz. Quite apart from death and destruction, the global economic impact would be incalculable.

All reason and common sense militate against such an act of aggression. Meir Dagan, the former head of Israel's Mossad, said last week it would be a "catastrophe". Leon Panetta, the US defence secretary, warned that it could "consume the Middle East in confrontation and conflict that we would regret".

There seems little doubt that the US administration is deeply wary of a direct attack on Iran. But in Israel, Barak has spoken of having less than a year to act; Binyamin Netanyahu, the prime minister, has talked about making the "right decision at the right moment"; and the prospects of drawing the US in behind an Israeli attack have been widely debated in the media.

Maybe it won't happen. Maybe the war talk is more about destabilisation than a full-scale attack. But there are undoubtedly those in the US, Israel and Britain who think otherwise. And the threat of miscalculation and the logic of escalation could tip the balance decisively. Unless opposition to an attack on Iran gets serious, this could become the most devastating Middle East war of all.

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