DONALD TRUMP’S political career arguably began with the full-page advertisements he purchased in The Post and several other newspapers in September 1987 in order to denounce Saudi Arabia, along with Japan, for “taking advantage of the United States.” Mr. Trump suggested, among other things, that the United States had no business defending the Persian Gulf, “an area of only marginal significance” for U.S. interests. He said the Saudis should “pay for the protection we extend as allies.”

Through the following three decades, including his presidential campaign, Mr. Trump stuck to that message. Yet in his inaugural foreign excursion as president Mr. Trump will touch down first not in Ottawa or London, but Riyadh, where he will celebrate what is being described as a renaissance of the U.S.-Saudi relationship. Mr. Trump is expected to announce enhanced U.S. support for the kingdom and its Gulf allies, including help with the formation of a defense alliance that U.S. officials say could evolve into an “Arab NATO.” The administration has also signed off on upward of $100 billion in arms sales to Saudi Arabia and the United Arab Emirates, including guided munitions that were held up by the Obama administration.

This upgraded alliance might be described as a roundabout response to Mr. Trump’s old critique. Administration officials say the massive Saudi purchases, though not a direct payment for U.S. defense, will create jobs in the United States and advance the day when the Persian Gulf states can defend themselves. Mostly, however, Mr. Trump’s sudden embrace of a regime he once excoriated reflects the success of an assiduous Saudi lobbying campaign. The kingdom’s seasoned diplomats set out to persuade the inexperienced Trump team that Saudi Arabia was an invaluable ally against the Islamic State as well as Iran — and appear to have succeeded all too easily.

The U.S. alliance with Saudi Arabia is worth preserving: Mr. Trump’s 1987 critique was way off base. But the administration’s new fervor for the tie appears to lack the healthy skepticism the Obama administration developed about Saudi enmity toward Shiite Iran, which appears to be driven by sectarian as well as strategic considerations. Iran’s aggression in the region should be resisted, but that should not mean depriving Shiites in Saudi Arabia, Bahrain and elsewhere of legitimate rights.

Senior U.S. officials also appear dangerously blasé about the Saudi military intervention in Yemen, an unwinnable war that has led to the deaths of thousands of civilians in bombings and has brought the country to the brink of famine. Riyadh is seeking more U.S. aid for the conflict, but humanitarian officials are deeply concerned about a pending Saudi assault on the port of Hodeida, through which the bulk of Yemen’s food supplies now flow. If it goes forward, mass starvation would be the likely result.

Defense Secretary Jim Mattis said during a visit to Saudi Arabia last month that the administration sought a political solution in Yemen, but he did not publicly oppose an offensive against Hodeida. A stand-down there ought to be the first return on the revamped relationship Mr. Trump has bought into. Otherwise, Saudi Arabia really will be taking unfair advantage of the United States.