Iran sits astride the Strait of Hormuz, a narrow waterway through which some 20 percent of global oil passes each day. In recent years, the regime has repeatedly attempted to manipulate U.S. policy with threats to close the strait — an action that, in theory, could send oil prices skyrocketing and wreak havoc on the world economy. In practice, however, history suggests that Iran’s gambit — if indeed Iran is behind the attacks — would almost certainly fail.

Blocking oil access rarely works as a strategy

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Historically speaking, the oil weapon rarely achieves its desired political objectives, especially against very powerful countries like the United States. My research has found that countries tried to disrupt oil access to great powers only seven times in the past century. Five of those seven attempts failed outright to achieve their political goals.

The two cases of success occurred under exceptional circumstances that Iran could not hope to replicate today. But those cases do have a common denominator: The United States was the coercer, not the target.

Arguably the clearest case of success — Japan’s surrender at the end of World War II, without an Allied invasion of the home islands — resulted from an overwhelming combination of pressures, including cutting off access to oil. By March 1945, the Allied blockade had completely severed Japanese oil imports, immobilizing Japan’s military and crippling its defenses against a U.S. invasion.

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Tokyo’s last chance was that the Soviets would accept an oil-for-territory deal that had languished on the negotiating table over the summer of 1945. Japan offered to withdraw from Manchuria in exchange for continued Soviet neutrality and Soviet oil. When Stalin instead invaded Manchuria on Aug. 9, all hope of fueling the home islands’ defense collapsed. That same day, the United States dropped its second atomic bomb on Japan. Oil denial influenced Japan’s choice to surrender, but the Soviet declaration of war, as well as the atomic bombings of Hiroshima and Nagasaki, likely mattered as much or more.

The other oil coercion example occurred in 1956, when Egypt, Syria and the United States persuaded Britain and France to withdraw from the Suez Canal Zone. The two countries had foolishly invaded Egypt to annex the Suez Canal, through which two-thirds of British oil imports traveled, for fear that Egyptian President Gamal Abdel Nasser might decide to close it.

The invasion not only failed to capture the canal — it also provoked Nasser to block the waterway and Syria to cut off the main oil pipeline to Europe. Furious, President Dwight D. Eisenhower refused British and French emergency requests for U.S. petroleum — to let them “boil in their own oil,” he remarked. At the same time, the crisis precipitated a run on the British pound. Oil denial likely contributed to the Anglo-French decision to stand down, but compared with financial pressures, it was probably less important.

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The oil weapon is hard to execute successfully

As these cases suggest, only under truly exceptional circumstances can coercers succeed in changing a great power’s policies through oil denial.

One reason is that depriving great powers of oil is very difficult, militarily. It took a full-scale conventional war for the United States to cut off oil to Japan. Iran, of course, cannot hope to do the same. Scholars have found that Iran’s ballistic missile capabilities pose little threat to neighboring countries’ oil industries, for example.

Likewise, Caitlin Talmadge has shown that Iran’s military is too weak to fully block oil commerce through the Strait of Hormuz. At best, Iran might depress tanker transit by harassing oil vessels enough to scare some shippers away, though scholars disagree over how much oil commerce an Iranian harassment campaign could suppress.

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History suggests that such a campaign would have limited effects on shipping and would not cause prolonged increases in oil prices. Tankers are remarkably difficult to sink. Even during the worst days of the 1980-1988 Iran-Iraq War, attacks by both sides on enemy tankers disrupted less than 2 percent of total oil traffic in the Persian Gulf. The result? Despite the attacks, oil prices actually fell dramatically across the span of the conflict.

The U.S. and its allies can avoid or mitigate the damage

And there’s another reason the oil weapon seldom works. Powerful nations take countermeasures to avoid being targeted in the first place, or to mitigate the effects of an attack.

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The United States already maintains a significant military presence in the Persian Gulf to deter threats and to respond quickly, if necessary, to clear the Strait of Hormuz.

Moreover, the United States and its allies maintain a network of strategic petroleum reserves to cushion any disruption of supply. Globally, these reserves top 4 billion barrels — enough oil to compensate for a complete closure of the Strait of Hormuz for up to eight months. That’s a more than adequate cushion: Iran cannot completely close the strait, and even pessimistic analyses conclude that the U.S. Navy could reopen the strait in about a month.

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More details may yet emerge about last week’s tanker story. But if Iran was involved, several motives could be at work, such as warning the United States to “back off” any threats of military intervention or persuading the United States to drop the sanctions the Trump administration recently reimposed over Iran’s nuclear program. Whatever Tehran might hope to accomplish, history suggests it probably won’t succeed.