(CNN) It shouldn't be difficult for the White House to pick between Saudi Arabia and Iran.

After all, the desert kingdom, now locked in an escalating showdown with Tehran that threatens to send the Middle East spiraling further into an abyss of instability, is a steadfast ally long at the center of American policy toward the region.

America, meanwhile, has waged a diplomatic, strategic and covert struggle with Iran since the Islamic Revolution in 1979. It has branded the country a state-sponsor of terror and is frequently blasted in Iran as the "Great Satan."

Yet Washington has done everything it can to avoid coming down too strongly on one side or the other in the current showdown, which has seen Riyadh and Tehran break off diplomatic relations, freeze trade and travel links, broaden a regional proxy war and engage in a public relations battle over who is to blame for the region's torment.

"We're of course concerned by any of these escalating tensions and the potential for an impact," said Pentagon spokesman Peter Cook earlier this month. "We would ask for cooler heads to prevail here and for both sides to do what they can to try and lower the tension level."

Changing regional dynamics

The measured American response is more than just another instance of the Obama administration trying to keep its distance from the Middle East. It's a sign of changing power dynamics in a region that has been reordered by a hardening of sectarian lines and the tumult unleashed by civil wars in Syria and Iraq and the rise of ISIS.

That reordering has deeply alarmed the Saudis, who have long feared a rising Iran and have lost trust in the United States in recent years, particularly as the Iran nuclear deal was clinched.

On Friday, Secretary of State John Kerry will confront Saudi disquiet face-to-face when he arrives for a visit to Riyadh and seeks to reassure the kingdom, which has recently experienced a leadership transition.

Yet so far, the U.S. reaction to the Saudi-Iran showdown has been characterized more by a desire to halt further escalations rather than jump into the fray to back an old ally or try to steer toward a certain outcome.

The U.S. initiative to engineer a political transition in Syria -- as far-fetched as it may seem as the vicious civil war grinds on -- depends on the Saudis and the Iranians buying into the concept. And few expect the threat of ISIS to be defused without stabilizing Syria, where chaos has allowed the terror organization to flourish.

In his comments, Cook spoke of the need for de-escalation between Iran and Saudi Arabia because such confrontation is "not conducive to the important fight that needs to be waged against (ISIS)."

Washington also appears intent on avoiding alienating more moderate elements in the Iranian government in order to preserve the nuclear pact, reached in July, that will form the centerpiece of President Barack Obama's foreign policy legacy.

Even Iran's capture of 10 U.S. sailors who had drifted into Iranian waters last Tuesday didn't derail the deal, with the U.S. quickly securing their release. Indeed, implementation of the deal went ahead as scheduled this weekend, at the same time that five Iranian-Americans were freed from detention in Iran as part of a prisoner exchange.

"The secretary is very concerned with the direction this thing is going," one senior official said of the Iran-Saudi escalation. "It's very unsettling to him that so many nations are choosing not to engage. With so much turmoil in the region, the last thing we need is for people not to be having conversations."

The U.S. position, however, is complicated by the fact that American influence with the Saudis is not what it once was, given the intense strain introduced into the relationship during the Obama presidency.

Riyadh alarmed

The government in Riyadh is simultaneously deeply alarmed at what it views as hegemonic Iranian activity throughout the Middle East and its belief that the Obama administration has failed to assess the true scope and nature of the threat.

"What we are seeing going on there is to some extent what happens when U.S. allies go off playbook when they don't think the U.S. is appropriately engaged with tackling the challenges of the region," said David Weinberg, a Saudi expert at the Foundation for the Defense of Democracies.

The crisis began earlier this month when Sunni-ruled Saudi Arabia executed Shiite cleric Nimr al-Nimr and 46 other men. The killings were followed by the storming of the Saudi embassy in Tehran, where the cleric had many adherents, and the situation deteriorated a few days later when Iran accused the Saudis of deliberately targeting its embassy in an airstrike in Yemen, where the two nations are on either sides of a civil war. From there, official relations, trade and travel ties were scrapped, with several of Saudi Arabia's Gulf allies following suit.

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U.S. officials have made clear that they advised the Saudi government not to execute Nimr. But the Saudis went ahead anyway, fueling a perception in Washington that the move was intended partly as a deliberate effort to prod Iran and to highlight the erosion of U.S. sway over the government in Riyadh.

For the Saudis, however, every calculation is colored by a fervent preoccupation with the intentions of Iran.

"I think the Saudis are habitually paranoid about Iran and particularly at this stage, because they think the Iranians are playing games," said Simon Henderson, a Saudi expert at the Washington Institute for Near East Policy.

Saudi leaders think that "the U.S. is being foolish in tolerating this in the name of a (nuclear) deal which they don't think is a very good deal," he said.

It is not the first time that the Saudi leaders and the United States have been at odds in recent years.

Relations deteriorated sharply in the aftermath of the Arab Spring, when Saudi royals looked on in dismay as Washington withdrew support for another bedrock of its Middle East policy, Egyptian President Hosni Mubarak.

Saudi-U.S. disconnect

That episode reflected a disconnect between the way that the White House and the Saudis view the region, which has at its heart a disagreement both on the political evolution of democracy and on how to manage Iran. Saudi Arabia's record on human rights and women's rights is often a cause for controversy in the United States.

In recent months, officials in Washington have also become disappointed with the scale of the Saudi involvement in the coalition fighting ISIS, as Riyadh has turned its attention to its military intervention in Yemen.

But from the Saudi point of view, Iran's nuclear program was just one aspect of a multipronged threat from Iran, said Henderson.

"Saudi Arabia and the Gulf states have been very concerned that this deal involved too much American compromise because the administration was so desperate for it," said Henderson.

Vali Nasr, a former senior State Department official in the Obama administration, spoke to the Saudi Arabian focus on Iran.

"The Saudis stated for them the most important issue is Iran and Iranian influence," Nasr told CNN in an interview.

"In fact, they explained the rise of ISIS and the problem in Syria only in terms of the Iranian threat," he continued. "The U.S. and Europe are worried about refugees and ISIS, and Saudi Arabia is worried about Iran."

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U.S. officials admit they are not sure whether the escalation of the crisis with Iran is motivated at least in part by Saudi Arabia's desire to show frustration with the Iran deal, or even an attempt to derail it entirely, or whether either side is using the crisis to jockey for position in Syria and across the wider region.

The official noted, however, that the fact that Sunni states allied to Saudi Arabia are severing ties with Iran helps opponents of the agreement in the United States argue that the rest of the region is not on board with it.

Senior Iranian and Saudi officials have now taken to playing out their dispute over the airwaves.

"We don't have confidence in Iran, we have confidence in the United States," Saudi Foreign Minister Adel al-Jubeir told CNN's Wolf Blitzer in an interview on Tuesday, saying Iran should stop supporting terrorism and militias who destabilize countries in the region and halt negative propaganda.

"Other than that, things should be fine with Iran," he concluded.

In a dueling interview with CNN's Christiane Amanpour on Wednesday, Iranian Foreign Minister Mohammad Javad Zarif appeared to play on Saudi fears, accusing the kingdom of an irrational overreaction and saying that Tehran sought cooperation.

"We do not have a fight to pick with Saudi Arabia," he said. "Unfortunately, the fact is that the instability in our region is caused by a panic in Saudi Arabia."

Syria in the balance

But Kerry has insisted that efforts to bring peace to Syria, at least, would not suffer from the dispute between Saudi Arabia and Iran, and has said that they have vital common interests that could keep the diplomacy alive as questions of whether a political transition away from the leadership of President Bashar al-Assad looms.

"What they differ on, obviously, is how you will resolve the actual Assad-Syria problem, but they don't differ on wanting a united Syria, a Syria that is stable, a Syria that is peaceful, a Syria that resolves this problem," Kerry said in Davos, Switzerland, on Thursday. "And that's why they've agreed to come to the table to put it to the test."

Still, the attempt to broker a Syrian transition remains an extreme long shot.

"The process that Secretary of State Kerry has been working very hard to put together was poised for failure before this Gulf spat, it is poised for failure after this Gulf spat," Weinberg said.

To keep even slim hopes of success alive, Kerry will have to convince the Saudi government -- just as it must also reassure the Israeli government -- that its fears that the recent dialogue between Iran and Washington, which would have seemed unthinkable for the past four decades, does not threaten its special ties with the United States.

"The Saudis and Israel believe this is a strategic mistake to engage Iran. But I don't agree. They should want us to have an in-depth understanding of what Iran is thinking," Nicholas Burns, a veteran diplomat who served in the No. 3 post at the State Department, told CNN. Burns this week signed a statement supportive of Democratic candidate Hillary Clinton's foreign policies.

"The Saudis should be assured that on every issues the U.S. and Iran are on the opposite sides of the divide," he added. "They don't need to worry about a strategic realignment. They should understand that we are with them"