Saudi Arabia is a natural stop on what may turn out to be US President Barack Obama's last big international tour.
The close alliance between the world's sole superpower and the Gulf oil-producing heavyweight has been a defining factor in Middle Eastern politics and US regional strategy for decades.
But the US leader's visit, during which he will have a one-on-one meeting Wednesday with Saudi King Salman and a summit with Gulf leaders on Thursday, comes amid mounting mutual frustration between Washington and Riyadh.
That frustration was voiced by Obama himself in a lengthy interview published by The Atlantic magazine earlier this month - a piece that did nothing to smooth over the two countries' diverging priorities in the region.
Obama, his interviewer Jeffrey Goldberg wrote, "is clearly irritated that foreign-policy orthodoxy compels him to treat Saudi Arabia as an ally."
Salman has shifted the country's foreign policy approach since he succeeded the late King Abdullah last year - with a more combative stance focused on the perceived threat from the kingdom's archrival for regional influence, Iran.
And that is precisely the area where the kingdom most feels a lack of support from Washington.
Saudi Arabia is still smarting over Obama's decision to back down from airstrikes against Iran's ally, Syrian President Bashar al-Assad, after a chemical weapons attack on a rebel-held area of Damascus in 2013 that killed hundreds.
The kingdom has long been frustrated by what it sees as Washington's overly cautious backing for Syrian rebels.
The deal struck over Iran's nuclear programme last year, under which the US backed the gradual lifting of international sanctions against the Islamic Republic, did nothing to reassure Riyadh.
In March 2015, when Iran-allied Houthi rebels in Yemen marched south towards the last holdouts of the Saudi-backed government, Salman took action himself, launching an air campaign backed by Gulf land forces.
By contrast, Obama told Goldberg that the Saudi-Iranian rivalry "requires us to say to our friends as well as to the Iranians that they need to find an effective way to share the neighbourhood and institute some sort of cold peace."
That went down badly in Riyadh.
Influential Saudi commentator Jamal Khashoggi says that the visit will be an opportunity for Obama "to clarify, and also to listen to the Saudis and the Gulf about why it is hard for Saudi Arabia to cooperate with Iran."
Meanwhile, ever since the al-Qaeda attack that brought down the twin towers of the World Trade Center in New York in 2001, voices have been raised from time to time in Washington suggesting that Saudi Arabia, which follows a hardline school of Islam sometimes called Wahhabism, is far from a natural ally for the US.
The Atlantic suggested that Obama has some sympathy for that view: In a private meeting with Australian Prime Minister Malcolm Turnbull, it reported, he cited Saudi money and clerics as a factor in growing Islamic fundamentalism in countries such as Indonesia.
"We Saudis ourselves practice this criticism and there is a revision of the role of the supporters of extremism in the region," Khashoggi said. "But also the Salafism practiced in some of the poor districts in Europe is not our Salafism as some believe."
Analyst Perry Cammack of the Washington-based Carnegie Endowment for International Peace says there are "deep structural changes" in the US relationship with the Gulf monarchies.
With its growing exploitation of its own energy resources, including the shale oil industry, the US is no longer as dependent on Gulf oil. Regional wars and the long-term impact of 9/11 have also altered the equation.
There will be no going back to the 1980s - when Riyadh's fierce opposition to communism and radical Arab nationalism made it a natural strategic partner for the US - Cammack argues.
From Riyadh's perception, too, there is a genuine shift in the relationship that's not just down to Obama, Khashoggi says. But he argues that it may be one that actually suits both parties as the US loses its appetite for direct intervention in the Middle East.
"The conditions and bases of the relationship have changed with King Salman's coming to power," Khashoggi argues. "Saudi Arabia has become more self confident and readier to take the initiative. This also suits America's wishes."
Riyadh, he says, would be happy to hear Obama express support for Saudi leadership in the region.
By Pol O Gradaigh
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