Saudi Arabia's intervention in Bahrain amid Shiite-led opposition violence has exposed festering political differences between Riyadh and the United States over the revolts rocking the Arab world.
The surprise Saudi decision to lead a regional mission into the strife-torn and strategic kingdom ruled by a Sunni minority also reflected the deep shadow cast by Iran in instability testing US-allied leaders across the Gulf.
Washington appeared to have little if any advance notice that Saudi Arabia, a crucial ally, would roll with Gulf Cooperation Council troops into Bahrain -- despite the visit of Defense Secretary Robert Gates to Manama last week.
A Pentagon spokesman said Monday that Washington was not forewarned about the plan. Other senior officials modified that stance on Tuesday, saying Washington was aware of the action, but not "consulted" about it.
But the fact that Saudi Arabia would take a step certain to embarrass the United States following Washington's prolonged effort to prod Bahrain towards political reform revealed the tense nature of bilateral relations.
Analysts said that the disagreement between the allies over Bahrain was a symptom of wider Saudi disgust at Washington's support for the concept of "universal rights" as unrest and rebellion sweep the Arab world.
"It is quite apparent that the United States and Saudi Arabia are not on the same page," said Simon Henderson, a specialist in Saudi and Gulf issues with the Washington Institute for Near East Policy.
David Ottaway, of the Woodrow Wilson International Center for Scholars agreed, saying "they are really upset by our pressing for democracy across the Arab world.
"I think our relationship with Saudi Arabia is going to remain very tense for the next few months and maybe longer."
Saudi frustration with Washington appears partly rooted in President Barack Obama's treatment of ousted Egyptian president Hosni Mubarak.
The Arab strongman may have been a staunch US ally for decades, but Washington turned its back as pro-democracy protests reached a critical mass.
So, Riyadh's dispatch of more than 1,000 troops to help Bahrain quell protests appeared to repudiate Obama's warning that regional leaders "can't maintain power through coercion" and risked being "behind the curve" of change.
The Saudis also appeared motivated by concern that a Shiite rebellion in Bahrain could be exploited by Iran to sow further revolt and instability and roil Shiite minorities in its own territory.
Washington took care to avoid a public spat with Saudi Arabia on Tuesday over Bahrain, a nation which hosts the headquarters of the US Fifth Fleet.
But its rhetoric reflected rising US anxiety on a day when Bahraini Shiite clerics said they feared a "massacre" of protesters in Manama and 200 Shiites were wounded in gang shootings.
National Security Council spokesman Tommy Vietor warned there was no "military solution" to Bahrain's problems, while saying the use of "force and violence" would only worsen the situation.
In Cairo, US Secretary of State Hillary Clinton apparently sought to bridge gaps with the Saudis, acknowledging that Bahrain had the right to seek GCC assistance, while calling on its Sunni government to negotiate a solution.
Obama has insisted that Washington must support aspirations of Arab peoples for universal rights, but must not itself intervene to prevent under pressure autocrats from branding revolts as client uprisings of the United States.
That stance has been insufficient for critics who want a proactive US role to push out teetering rulers in places like Libya.
But some Arab leaders appear unimpressed for the opposite reason, seeing Obama's stance as interference, overly idealistic and a rejection of long-time US strategic assumptions and allies, analysts said.
"The White House enthusiasm for universal rights is a odds with not only the way many countries in the Middle East run themselves, but also completely at odds with what the Saudis and Bahrainis regard as an appropriate pace of potential change," said Henderson.
US officials privately admit the two sides are at odds over the concept of reform in the Middle East.
But they argue that the bedrock of a relationship that has underpinned US diplomacy in the region for decades -- and on which much of the global economy relies through Saudi oil exports -- remains firm.
American officials still have a close relationship with their Saudi counterparts on counterterrorism issues.
For example, Saudi Arabia is believed to have behind a tip-off which alerted Washington to bombs placed on US-bound cargo planes last year.
And final negotiations have been taking place on a massive $60 billion US arms sale to Saudi Arabia.
The deal has brought the two militaries closer together than they have been for three decades, Ottaway said.
"But on the political issue we are further apart than we have been for three decades," he said.