This time, several things postponed a price collapse. One was the growing consumption in the developing world, led by China. Another was turmoil in Libya, South Sudan and other countries that reduced supply. Over a million barrels per day were also taken off the market by sanctions imposed on Iran. Without that big surge of shale oil from the United States, it is highly likely that those sanctions would have failed. Prices would have spiked, countries seeking cheaper oil would have broken ranks — and Iran might not be at the nuclear negotiating table today.

By the middle of last summer, however, circumstances changed. World economic growth was slowing. Europe was on its back. Since 2004, China’s rapidly rising demand for oil had been the defining factor for the global oil market. But China’s economy was slowing, too, and that meant slowing growth in oil demand. At the same time, Libya managed to quadruple its oil output, at least for a time. That was the trigger for the beginning of the price decline.

It was assumed that OPEC would step in and cut production to boost the price. Trillions of dollars of investment have been made over the last several years on that premise.

But Saudi Arabia and the other gulf countries declined to do so. If they reduced production, they reasoned, they would lose market share permanently. As they saw it, they would be cutting back on their “low cost oil” to make room for “high cost oil,” and then would have to make more cuts to accommodate more high cost oil.

They were looking at competition not only from American shale oil but also from Canadian oil sands and new supplies from Russia, the Arctic, Brazil, Central Asia, Africa and growing volumes of offshore oil around the world.

But, most immediately, they were looking at two neighbors. They did not want to give up markets to Iraq, a country they see as an Iranian satellite, and whose output is increasing. And they certainly did not want to make way for Iran, which they thought might come to a nuclear deal with the United States and its allies, bringing that missing Iranian oil back into the market.

The depth of the price fall may be much more than even some of the gulf producers anticipated. Around the world, oil companies are cutting budgets, paring costs, slowing down projects and postponing new ones. Some may end up being canceled.