President Trump vacillated but decided on Sunday not to join Saudi Arabia in joint strikes against the Houthis in Yemen. For two weeks, the question of whether to yield to pleas from Saudi Arabia and order the U.S. military to strike targets in Yemen controlled by the Iran-backed Houthis has bedeviled President Trump. A decision to go forward could open another front in the war against Iran that Mr. Trump started but has been unable to end as it spreads economic shock around the globe.

The president had wavered for weeks over the Saudi appeals for help in fighting the Houthis, who took advantage of the instability in the Middle East and reignited their war with the kingdom this summer. But things came to a head on Thursday: Crown Prince Mohammed bin Salman, the kingdom's de facto leader and a major ally in the Iran war, called the president to implore him, again, to step in to help blunt the rapidly advancing group.

Mr. Trump's decision to pull back from, or at least delay, striking Houthi targets came as commanders had approved target lists, troops were loading bombs onto U.S. warplanes selected for the missions and the wave of attacks was just about to begin. The gyrations reflect the quandary in which the president now finds himself: He is correct when he says that the Houthis have steered clear of attacking U.S. ships. But they have gone after Saudi Arabia, creating a dilemma for Mr. Trump.

Some U.S. military officials have argued that the United States needs the kingdom's help with the war in Iran, and Saudi officials have shown they are willing to exercise what leverage they have in the absence of U.S. cooperation. For instance, in May, Saudi Arabia effectively halted the Trump administration's plan to use the U.S. military to escort commercial oil tankers through the Strait of Hormuz by refusing the United States use of its airspace and American air bases in the country.