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Saudi Arabia reduced daily oil production by about 5.7 million barrels after strikes against facilities in the country’s east on Saturday. Iran-backed Houthi rebels claimed credit, though Secretary of State Mike Pompeo tweeted that Iran was responsible and there was “no evidence the attacks came from Yemen.” Iran denies this, but it usually uses proxies to avoid a direct confrontation and there are no other plausible culprits.

This is more than a local dispute between two regional powers. The attacks have caused a roughly 5% reduction in global daily oil production. The Saudis have promised to dip into reserves to offset the losses, but oil prices could rise and harm an already fragile global economy if the Kingdom isn’t able to restore production fast enough.

American shale oil production can take up some of the slack but that would take time. Long-term damage to oil supplies would increase the pressure on the U.S. to ease sanctions on Iranian oil exports, which Mr. Trump has been considering.

The attack continues what is already a hot proxy war between Iran and Saudi Arabia, an important U.S. ally. The extent of the damage raises doubts about how well the Saudis can defend against future drone assaults. Saudi intelligence and air defenses don’t seem up to the job. Saudi revenues would be hurt by a reduction in oil output, and uncertainty will complicate an initial public offering of the country’s national oil company, Aramco.

Even if the Houthis didn’t carry out this attack, Iran is backing their war against an Arab coalition in Yemen. The Houthis have become increasingly aggressive in attacking sites in Saudi Arabia and oil tankers in the Red Sea. If the Saudis cede Yemen to the Houthis, Iran will have won another proxy war, this one on the Arabian peninsula. The Saudis are far from ideal allies, but U.S. Senators who want to end U.S. support for Riyadh should consider the alternative of Iranian regional dominance.

The White House says Mr. Trump spoke with Saudi Crown Prince Mohammed bin Salman and pledged U.S. support. But the White House should be contemplating more than words.

The Iranians are probing Mr. Trump as much as the Saudis. They are testing his resolve to carry out his “maximum pressure” campaign, and they sense weakness. Iran shot down an American drone this summer, and Mr. Trump rejected advice for a military response. Qassem Soleimani, commander of Iran’s overseas Quds Force, has historically interpreted such restraint as a signal that he’s winning and can safely escalate.

Mr. Trump is also eager for direct talks with Iranian President Hassan Rouhani, and Mr. Pompeo floated a handshake meeting between the two at the coming United Nations General Assembly. Mr. Trump has even contemplated support for French President Emmanuel Macron ’s idea of paying the mullahs a $15 billion bribe for better behavior. The weekend attack is Iran’s return handshake.

U.S. sanctions have hurt Iran’s crude oil exports, but Tehran still earns hundreds of millions of dollars a month from other petroleum products. Senator Lindsey Graham says direct attacks on Iranian oil production should be considered, and the Islamic Republic needs to know that is not off the table.

The Saudi coalition also needs more help interdicting Iranian arms shipments to the Houthis. Americans are understandably wary of deeper involvement in Yemen, but a victory for Iran and the rise of a Hezbollah-like regime in Sana’a will harm U.S. security interests. Think another Syria and Lebanon.

Mr. Trump might also apologize to John Bolton, who warned repeatedly that Iran would take advantage of perceived weakness in the White House. Mr. Bolton resigned last week over policy differences, notably on Iran. The weekend’s events proved the former adviser right. The Trump Administration’s pressure campaign has been working, and abandoning it now would encourage Tehran to take more military risks.