THE collapse of the Soviet Union in 1991 had many causes. None was as basic as the fall in the price of oil, its main export, by two-thirds in real terms between 1980 and 1986. By the same token, the 14-year rule of Vladimir Putin, heir to what remained, has been bolstered by a threefold rise in the oil price. Now the oil price is falling again. Since June, it has dropped from about $115 for a barrel of Brent crude to $85 or so—a reduction of roughly a quarter. If prices settle at today’s level, the bill for oil consumers will be about $1 trillion a year lower. That would be a shot in the arm for a stagnating world economy. It would also have big political consequences. For some governments it would be a rare opportunity; for others, a threat.

The scale of shale

Predicting oil prices is a mug’s game (we speak from experience). The fall of the past three months is partly the result of unexpected—and maybe short-lived—developments. Who would have guessed that chaotic, war-torn Libya would somehow be pumping 40% more oil at the end of September than it had just a month earlier? Saudi Arabia’s decision to boost output to protect its market share and hurt American shale producers and see off new developments in the Arctic was also a surprise. Perhaps the fall was exaggerated by hedge-fund investors dumping oil they had been holding in the false expectation of rising prices.

Geopolitical shocks can surprise on the upside as well as the down. Saudi Arabia may well decide to resume its self-appointed post as swing producer and cut output to push prices up once more. With war stalking Iraq, Libya still fragile and Nigeria prey to insurgency (see article), supply is vulnerable to chaotic forces.

But many of the causes of lower prices have staying power. The economic malaise weighing down on demand is not about to lift, despite the tonic of cheaper oil (see article). Conservation, spurred by high prices and green regulation, is more like a ratchet than a piece of elastic. The average new car consumes 25% less petrol per mile than ten years ago. Some observers think the rich world has reached “peak car”, and that motoring is in long-term decline. Even if they are wrong, and lower prices encourage people to drive more, energy-saving ideas will not suddenly be uninvented.

Much of the extra supply is baked in, too. Most oil investment takes years of planning and, after a certain point, cannot easily be turned off. The fracking revolution is also likely to rage on. Since the start of 2010 the United States, the main winner, has increased its output by more than 3m barrels per day to 8.5m b/d. Shale oil is relatively expensive, because it comes from many small, short-lived wells. Analysts claim that a third of wells lose money below $80 a barrel, so shale-oil production will adjust, helping put a floor under the price. But the floor will sag. Break-even points are falling. In past price squeezes, oilmen confounded the experts by finding unimagined savings. This time will be no different.

For governments in consuming countries the price fall offers some budgetary breathing-room. Fuel subsidies hog scandalous amounts of money in many developing countries—20% of public spending in Indonesia and 14% in India (including fertiliser and food). Lower prices give governments the opportunity to spend the money more productively or return it to the taxpayers. This week India led the way by announcing an end to diesel subsidies. Others should follow Narendra Modi’s lead.

The axis of diesel

For those governments that have used the windfall revenues from higher prices to run aggressive foreign policies, by contrast, things could get uncomfortable. The most vulnerable are Venezuela, Iran and Russia.

The first to crack could be Venezuela, home to the anti-American “Bolivarian revolution”, which the late Hugo Chávez tried to export around his region. Venezuela’s budget is based on oil at $120 a barrel. Even before the price fall it was struggling to pay its debts. Foreign-exchange reserves are dwindling, inflation is rampant and Venezuelans are enduring shortages of everyday goods such as flour and toilet paper.

Iran is also in a tricky position. It needs oil at about $140 a barrel to balance a profligate budget padded with the extravagant spending schemes of its former president, Mahmoud Ahmedinejad. Sanctions designed to curb its nuclear programme make it especially vulnerable. Some claim that Sunni Saudi Arabia is conspiring with America to use the oil price to put pressure on its Shia rival. Whatever the motivation, the falling price is certainly having that effect.

Compared with these two, Russia can bide its time. A falling currency means that the rouble value of oil sales has dropped less than its dollar value, cushioning tax revenues and limiting the budget deficit. The Kremlin can draw on money it has saved in reserve funds, though these are smaller than they were a few years ago and it had already budgeted to run them down. Russia can probably cope with today’s prices for 18 months to two years, but the money will eventually run out. Mr Putin’s military modernisation, which has absorbed 20% of public spending, looks like an extravagance. Sanctions are stifling the economy and making it hard to borrow. Poorer Russians will be less able to afford imported food and consumer goods. If the oil price stays where it is, it will foster discontent.

Democrats and liberals should welcome the curb the oil price imposes on countries like Iran, Venezuela and Russia. But there is also an increased risk of instability. Iran’s relatively outward-looking president, Hassan Rouhani, was elected to improve living standards. If the economy sinks, it could strengthen the hand of his hardline opponents. Similarly, a default in Venezuela could have dire consequences not just for Venezuelans but also for the Caribbean countries that have come to depend on Bolivarian aid. And Mr Putin, deprived of economic legitimacy, could well plunge deeper into the xenophobic nationalism that has fuelled his campaign in Ukraine. Cheaper oil is welcome, but it is not trouble-free.