Together they form an “Axis of Diesel”. Buoyed by petrodollars, Russia, Iran and Venezuela hectored the West as they extended their reach abroad, backing separatists in Georgia, Islamists in the Middle East and Leftists around the world.

Now those oil-producing powers may be forced to draw in their horns as crude prices tumble. They face austerity budgets that could force them to scale back their military spending and foreign assistance even as falling oil prices fuel domestic dissent.

“All countries heavily dependent on petroleum revenue are nervously watching oil prices as they drop not just far, but quickly,” said Jonathan Elkind, a senior Fellow at the Brookings Institution in Washington.

“That price adjustment is raising questions in all these capitals about the suitability of the economic model that has been making them feel so full of themselves in the recent period.

“It would be a serious mistake for people in the United States or other net consumers to feel a sense of the satisfaction that ‘Happy days are here again',” he said. “They're not.”

Leaders in Tehran, Moscow and Caracas have gloated as the financial crisis has hobbled the United States and its Western allies. Analysts say that the three swaggering petro-states are the most vulnerable oil producers to the steep price declines. From a record high of $147 (£85) a barrel in July, crude oil is now trading at around $70 after dipping to its lowest level since August 2007.

Deutsche Bank estimated in a recent research note that Iran and Venezuela need an oil price of more than $95 a barrel to balance their budgets, and Russia requires a price of $75. That compares to a break-even figure of $55 for Saudi Arabia.

Iran and Venezuela have led so-called oil hawks in recent days to push the producer cartel Opec to bring forward an emergency meeting to next Friday, from mid-November, to discuss cutting output quotas to drive up the price. While Russia has prudently salted away much of its oil windfall in “rainy day” funds, Iran and Venezuela are much worse prepared for the downturn, Mr Elkind said.

The tumbling oil prices are grim news for President Ahmadinejad of Iran as he prepares to fight for re-election next June. The populist son of a blacksmith won a landslide election victory three years ago by pledging to give the poor a fairer share of Iran's oil wealth. Now the economy is his Achilles' heel. His profligate spending of petrodollars from record oil revenues has stoked inflation, which topped 29 per cent last month, compared with 12 per cent when he took power.

Bazaar merchants - a potent middle-class force - went on strike last week for the first time since the run-up to the country's Islamic revolution, forcing Mr Ahmadinejad to scrap plans to impose 3 per cent VAT to help to replenish Iran's coffers.

Iranian reformers are urging the headstrong Mr Ahmadinejad to prepare for lower oil revenues by slashing subsidies on commodities such as sugar, cooking oil and wheat. Instead, with an eye on the elections, he continues to tour the provinces, attempting to buy rural support by dispensing largesse in cash and loans.

In Venezuela the Government has unveiled an austerity budget. Just as in Iran, however, Mr Chávez maintains his populist social spending ahead of municipal and state elections. Economic analysts predict that the Government will be forced to raise taxes and devalue the currency.

First affected may be Venezuela's foreign allies. The country's energy aid to friendly nations, which has bought it influence across the continent, is likely to be reined in. Its generous credit programme for Caribbean partners in the PetroCaribe energy accord has been reduced from 50 to 40 per cent.

Defence spending may also be hit. Venezuela has bought about $4.4 billion-worth of Russian military equipment since 2005. Last month it got a $1 billion Russian loan for more purchases - the first time it has sought financing for arms deals with Moscow.

Russia is best positioned for the crisis, having built up the world's third-largest foreign currency reserves before the crisis, at $580 billion. As its stock market plunged it has been forced to spend more than $32 billion in the past two weeks to prop up the currency and bail out banks. The Kremlin will be forced to plug holes in next year's budget by dipping into the Reserve Fund, a $154 billion repository of windfall oil revenues forecast to grow to $174 billion by 2010, but may now start to shrink instead.

President Medvedev, however, is determined to press on with modernisation of the military and has adopted an increasingly strident tone with the West. He has ordered a renovation of Russia's nuclear deterrent and the creation of new space and missile defence shields by 2020, as well as the “mass production of warships... and multi-purpose submarines”.

Nevertheless, Western diplomats detected signs of a new Russian flexibility during last month's UN General Assembly, when Moscow backed an extension of the Nato mandate in Afghanistan and agreed to a meeting on Iran's nuclear programme.

Frank Verrastro, of the Centre for Strategic and International Studies in Washington, noted that oil prices had only fallen to last year's levels and cautioned that it would take more sustained price falls to trigger long-lasting changes by major oil producers.

“It's premature to say people are radically changing their behaviour,” he said. “I think they will, but not yet.”

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