FOR the past half-century, the world economy has been held hostage by just one country: the Kingdom of Saudi Arabia.

FOR the past half-century, the world economy has been held hostage by just one country: the Kingdom of Saudi Arabia. Vast petroleum reserves and untapped production allowed the kingdom to play an outsize role as swing producer, filling or draining the global system at will.

The 1973-74 oil embargo was the first demonstration that the House of Saud was willing to weaponize the oil markets. In October 1973, a coalition of Arab states led by Saudi Arabia abruptly halted oil shipments in retaliation for America’s support of Israel during the Yom Kippur War. The price of a barrel of oil quickly quadrupled; the resulting shock to the oil-dependent economies of the West led to a sharp rise in the cost of living, mass unemployment and growing social discontent.

“If I was the president,” Secretary of State Henry Kissinger fumed to his deputy Brent Scowcroft, “I would tell the Arabs to shove their oil.” But the president, Richard M. Nixon, was in no position to dictate to the Saudis.

In the West, we have largely forgotten the lessons of 1974, partly because our economies have changed and are less vulnerable, but mainly because we are not the Saudis’ principal target. Predictions that global oil production would eventually peak, ensuring prices stayed permanently high, never materialised. Today’s oil crises are determined less by the floating price of crude than by crude regional politics. The oil wars of the 21st century are underway.

In recent years, the Saudis have made clear that they regard the oil markets as a critical front line in the Sunni Muslim-majority kingdom’s battle against its Shiite-dominated rival, Iran. Their favoured tactic of “flooding,” pumping surplus crude into a soft market, is tantamount to war by economic means: the oil trade’s equivalent of dropping the bomb on a rival.

In 2006, Nawaf Obaid, a Saudi security adviser, warned that Riyadh was prepared to force prices down to “strangle” Iran’s economy. Two years later, the Saudis did just that, with the aim of hampering Tehran’s ability to support Shiite militia groups in Iraq, Lebanon and elsewhere.

Then, in 2011, Prince Turki al-Faisal, the former chief of Saudi intelligence, told NATO officials that Riyadh was prepared to flood the market to stir unrest inside Iran. Three years later, the Saudis struck again, turning on the spigot.But this time, they overplayed their hand.

When Saudi officials made their move in the fall of 2014, taking advantage of an already glutted market, they no doubt hoped that lower prices would undercut the American shale industry, which was challenging the kingdom’s market dominance. But their main purpose was to make life difficult for Tehran: “Iran will come under unprecedented economic and financial pressure as it tries to sustain an economy already battered by international sanctions,” argued Mr. Obaid.

Oil-producing countries, especially ones like Russia, with relatively undiversified economies, base their budgets on oil prices not falling below a certain threshold. If prices plunge below that level, fiscal meltdown looms. The Saudis expected a sharp reduction in oil prices not just to hurt the American fracking industry, but also to hammer the economies of Iran and Russia. That in turn would weaken their ability to support allies and proxies, particularly in Iraq and Syria.

The tactic had been brutally effective in the past. This was the grim scenario that confronted the shah in 1977 when the Saudis flooded the oil market to rein in Iran’s influence. The 1977 flood was not the sole cause of the Iranian revolution, but it certainly was a factor: The shah’s rule was destabilised just as Ayatollah Ruhollah Khomeini mounted his offensive to replace a pro-Western monarchy with a theocratic state. In that sense, the oil markets fueled the rise of political Islam.

The price of oil also helped end the Cold War. Then, like Russia today, the Communist superpower was a global energy producer heavily reliant on revenues from oil and gas. In 1985-86, the Saudis’ decision to flood the market — which some believe was encouraged by the Reagan administration — led to a collapse in prices that sent the Soviet economy into a tailspin.

“The timeline of the collapse of the Soviet Union can be traced to Sept. 13, 1985,” wrote the Russian economist Yegor Gaidar. “On this date Sheikh Ahmed Zaki Yamani, the minister of oil of Saudi Arabia, declared that the monarchy had decided to alter its oil policy radically.”

Today, in Russia, fully half of government revenue comes from oil and gas. Even if oil returns to $40 a barrel — it twice fell below $30 earlier this year — that depressed price still creates “a dangerous scenario,” according to Mikhail Dmitriev, a former Russian deputy economic minister. Inflation in Russia hit double digits last year; its sovereign wealth fund, which bails out struggling Russian companies, is depleted; and factory closings are fueling labor unrest.

Unhappily for President Vladimir V. Putin, Russia’s fiscal crisis has coincided with his military interventions in eastern Ukraine and Syria. If Russia’s economy worsens and Mr. Putin feels cornered, he may look for ways to distract the Russian people with more rally-round-the-flag provocations, as well as induce panic in the oil markets about supplies and gin prices back up.

Future shock has already arrived for oil producers like Venezuela, whose economy has been gutted by lost revenues from oil, which makes up 95 percent of its export earnings. With inflation predicted by the International Monetary Fund to reach 720 percent this year, Venezuela has become a financial zombie state — a harsh reminder of what can happen to countries that rely so heavily on a single unstable commodity price. President Nicolás Maduro is at the mercy of the markets that, every day, nudge his tottering regime nearer the abyss.

Another oil producer, Nigeria, is running out of money, hobbling President Muhammadu Buhari’s campaign against the Islamist Boko Haram insurgents in the northeast. The plunge in oil prices has also shaken Central Asia, where Azerbaijan and Kazakhstan have expressed interest in emergency bailouts from the I.M.F. and other lenders.

In the Middle East, reduced oil revenues have restricted Iraq’s ability to wage war against the Islamic State. Persian Gulf oil producers like Qatar and the United Arab Emirates estimate collective losses of $360 billion in export earnings in the past year. Such a big budgetary hole poses problems with maintaining order at home while fighting wars in Syria and Yemen, and propping up cash-strapped allies like Egypt.

And then there is Saudi Arabia itself. All the evidence suggests that Saudi officials never expected oil prices to fall below $60 a barrel. But then they never expected to lose their sway as the swing producer within the Organisation of the Petroleum Exporting Countries, or OPEC. Despite wishful statements from Saudi ministers, the kingdom’s efforts last month to make a deal with Russia, Venezuela and Qatar to restrict supply and push up prices collapsed.

The I.M.F. has warned that if government spending is not reined in, the Saudis will be bankrupt by 2020. Suddenly, the world’s reserve bank of black gold is looking to borrow billions of dollars from foreign lenders. King Salman’s response has been to promise austerity, higher taxes and subsidy cuts to a people who have grown used to state largess and handouts. That raises questions about the kingdom’s internal cohesion — even as the king decided to shoulder the burden of regional security in the Middle East, fighting wars on two fronts. Has there ever been an oil state as overleveraged at home and overextended abroad?

Meanwhile, by concluding the historic nuclear agreement, Iran is getting out from under the burden of economic sanctions. It will not be lost on Riyadh that this adds another oil producer to the world market that it can no longer control.

The instability and economic misery for smaller oil-producing states like Nigeria and Azerbaijan look set to continue. But that’s collateral damage. The real story is how the Saudis have been hurt by their own weapon.

Andrew Scott Cooper is the author of “The Oil Kings: How the United States, Iran and Saudi Arabia Changed the Balance of Power in the Middle East” and the forthcoming “The Fall of Heaven: The Pahlavis and the Final Days of Imperial Iran.”

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