The oil price war Saudi Arabia launched against Russia sent crude prices into one of the steepest falls in history Monday, compounding depressed global demand from the coronavirus and causing casualties for oil field workers, U.S. shale drillers, investors and members of the Organization of the Petroleum Exporting Countries that rely on oil to make their budgets add up.

Moscow’s refusal to cut its oil output by a half-million barrels a day shattered the unusual three-year marriage of OPEC, led by the Saudis, and major non-OPEC producers, led by Russia, as oil producers scrambled to find a way to respond to weakening global demand resulting from the deepening crisis over the novel coronavirus.

Saudi Arabia, angered by Moscow’s position, said Sunday that it would open its spigots and drive down prices, making this oil price cycle the only one in nearly a century to combine weak demand with a global price war.

Global markets opened sharply down Monday, with the leading benchmark varieties of crude tumbling more than 20 percent after other big falls in recent weeks. The price of West Texas Intermediate benchmark dropped to $31.13 a barrel by the end of the workday, down 25 percent. The drop was the steepest since prices plunged 35 percent on Jan. 17, 1991, the day the U.S.-led coalition launched Operation Desert Storm to force Iraq to withdraw from Kuwait.

The plunge in prices, however, will also benefit consumers at the pump. For them, the price war will act like a tax cut, putting more money into the pockets of motorists. The $17-a-barrel drop in crude prices over the past five days would translate into a ­40- cent-a-gallon drop in gasoline if companies pass the savings along to motorists.

Battered U.S. airlines, which used 18.3 billion gallons of fuel in 2019 but have cut back flights this year, would get some modest relief.

“I consider this as a $1 trillion stimulus to the world economy with a $30 oil price decline,” Scott Sheffield, chief executive of the Irving, Tex.-based Pioneer Natural Resources, one of the biggest independent oil companies, said in an email.

President Trump weighed in on Twitter, noting that lower prices were “Good for the consumer, gasoline prices coming down!”

Sheffield said Pioneer is secure, but other companies plying the vast shale deposits in places such as North Dakota, Oklahoma and Texas will face difficulties.

“We are preparing for two years of low prices and will make the necessary adjustments to maintain our great balance sheet,” he said. But, he added, “there will be many bankruptcies in our industries and tens of thousands of layoffs over the next 12 months.”

Investors pummeled a broad range of energy stocks Monday and wiped out hundreds of billions of dollars of their market value. By midday, shares of Pioneer and Continental Resources were down 37 percent and 53 percent, respectively.

Trump’s ally and energy adviser Harold Hamm is the founder and executive chairman of Continental. Hamm owns 76 percent of the company and personally lost $2 billion on Monday.

Shares of the major international oil companies fell 12 to 19.5 percent, and oil field services giant Schlumberger slid 27.4 percent.

The slide highlighted the exposure of a handful of banks with large amounts of loans to energy firms, according to a report by Keefe, Bruyette and Woods, a financial services firm. Of the five with the biggest exposures to the sector, two are in Oklahoma, two in Texas and one in Kansas, the report said.

The steep fall in oil prices “will likely cause negative credit quality trends and energy related losses for those exposed,” the firm said. “Overall, we view energy as a near-term risk.”

BOK Financial Corporation of Tulsa was in the most “worrisome” position of any of the banks surveyed, with energy loans equal to 108 percent of one measure of capital called “tangible common equity.”

Stacy Kymes, the company’s executive vice president, said most of the bank’s borrowers are sufficiently hedged against this sort of falloff. And 42 percent are primarily in natural gas, which he doesn’t expect to suffer the same price hit.

“We certainly aren’t excited about the price decline,” he said. “But we believe it is manageable.”

Oil demand will decline for the first time since 2009, the International Energy Agency said Monday, revising its earlier forecasts. The IEA said that global demand will shrink by 2.5 million barrels a day in the first quarter of the year.

“This is the first time since 1930 and ’31 that a massive negative demand shock has coincided with a supply shock,” Robert McNally, president and founder of the Rapidan Energy Group, said in an email. “It’s sort of like a black swan to simultaneous hurricane event.”

Most of the places that will be hurt worst by the crash in prices will be the U.S. shale fields, where costs are substantially higher than the giant reservoirs of Saudi Arabia or Russia. Thanks to technological advances and untapped shale layers, U.S. oil and natural gas output has soared, insulating the U.S. economy somewhat and easing price pressures worldwide. Texas now produces more crude oil than Iraq, and production has also increased in states such as North Dakota and Oklahoma.

But the higher costs and the short lives of shale wells — usually 18 to 24 months — make the U.S. fields more sensitive to lower prices. Sheffield said he expects U.S. shale production to start declining in the third quarter of this year and to fall more than 1 million barrels a day in 2021.

Rystad Energy, an analytical firm in Norway, issued a report Monday titled “See you in June at $25 per barrel?” Below that price, the company said, American shale producers will have to consider mothballing wells already drilled but not yet completed.

“No question U.S. shale producers will experience collateral damage in this price war between Russia and Saudi Arabia,” McNally said. “They were already under enormous pressure, and supply growth was set to taper off sharply by the end of the year.”

This isn’t the first time Saudi Arabia has sought to punish other oil-exporting countries for expecting the kingdom to act as the sole swing producer, cutting its own output while other exporters benefit. In the 1980s, Saudi Arabia drove down prices to less than $10 a barrel.

Oil-exporting countries had agreed to production cuts of 2.1 million barrels a day that were due to expire at the end of March. At an OPEC meeting last week, those cuts were extended through the end of the year. But OPEC also proposed an additional 1.5 million barrels a day of cuts, with non-OPEC countries providing a half-million barrels a day of output cuts. Russia declined.