In the months since a missile and drone attack widely seen as the work of Iran left two Saudi oil facilities smoldering, the Saudi crown prince has taken an uncharacteristic turn to diplomacy to cool tensions with his regional enemies.

The prince, Mohammed bin Salman, has stepped up direct talks with the rebels he has been fighting in Yemen for over four years, leading to a decline in attacks by both sides.

Fueling the shift from confrontation to negotiation, analysts say, is the sobering realization that a decades-old cornerstone of American policy in the Middle East the understanding that the United States would defend the Saudi oil industry from foreign attacks can no longer be taken for granted.

Even though U.S. and Saudi officials agreed that Iran was behind the Sept. 14 attacks on the petroleum processing plants at Abqaiq and Khurais, temporarily halving Saudi Arabia’s oil production, President Donald Trump responded with heated rhetoric but little else.

For the Saudis, the tepid response drove home the reality that despite the tens of billions of dollars they have spent on American weapons more than $170 billion since 1973 they could no longer count on the United States to come to their aid, at least not with the force they expected.

“I think we will look at Sept. 14 as a seminal moment in Gulf history,” said David B. Roberts, a scholar of the region at King’s College London. With the presumption shattered that the United States would protect the Saudis, Roberts said, “they realize the need to be more accommodating.”

For the United States, the shift toward diplomacy is an awkward paradox. The Trump administration and Congress have been pressing the Saudis to end the war in Yemen, and the administration has pushed them to reconcile with Qatar, largely in vain. Now, the presumed Iranian strikes may have done more to advance those goals than American pressure ever did.

Saudi Arabia’s foreign policy turned more aggressive after Prince Mohammed, then 29, emerged as its driving force in 2015. He plunged the kingdom into a devastating war against Iranian-backed rebels in Yemen; imposed a punishing boycott on Qatar, which he accused of supporting terrorism and cozying up to Iran; and vowed to confront Iran across the Middle East.