US President Barack Obama lands in Saudi Arabia on Wednesday to intensify the struggle against militants and efforts to end wars in Syria and Yemen, while trying to ease tensions with Riyadh.
After his early-afternoon arrival in the Saudi capital, Obama is to hold talks with King Salman, 80, who has presided over a more assertive foreign policy since acceding to the throne last year.
On Thursday, the president will attend a summit with the six Gulf Cooperation Council monarchies, joined by US Defence Secretary Ashton Carter and Secretary of State John Kerry.
They will gather in an atmosphere of bitterness with regional leaders offended by Obama's tone and actions, particularly what they see as his reluctance to get involved in Syria and other regional problems, as well as his tilt towards Iran.
The Sunni Gulf monarchies are worried after the lifting this year of international sanctions against their regional rival, Shia Iran.
Riyadh and its neighbours fear the US-supported international deal to curb Iran's nuclear programme will embolden the country, which they accuse of interference throughout the Middle East.
Mustafa Alani, a senior adviser to the Gulf Research Center, said Obama's presidency has been "100 percent negative" for the region, a legacy of "keeping his distance".
But the White House has emphasised the strength of an alliance that has endured more than 70 years, seeking to minimise the frictions.
"There have always been complexities in the US-Saudi relationship. There's been a core to that relationship in which we cooperate on shared interests like counterterrorism," said Ben Rhodes, a close adviser to Obama, who is in the final months of his mandate.
Saudi-US ties are founded on an exchange of oil for security.
"The Saudis are frustrated. They have the impression, probably rightly so, that the White House does not really understand their concern of the Iranian threat", said Lori Plotkin Boghardt, of the Washington Institute for Near East Policy.
Saudis reacted with outrage to comments by Obama published in the April edition of US magazine The Atlantic.
He said the Saudis need to "share" the Middle East with their Iranian foes, saying that competition between Riyadh and Tehran has helped to feed proxy wars and chaos in Syria, Iraq and Yemen.
Obama spoke of "free riders", suggesting that certain states had not assumed their share of responsibility for regional security.
"I think the US has had long, longstanding concerns about the way the Saudis are behaving in the region. And this pre