Saudi Arabia kept some key details of its military action in Yemen from Washington until the last moment, US officials said, as the kingdom takes a more assertive regional role to compensate for perceived US disengagement.
The Middle East's top oil power told the United States weeks ago it was weighing action in Yemen but only informed Washington of the exact details just before Thursday's unprecedented air strikes against Iran-allied Houthi rebels, the officials said.
US President Barack Obama's Middle East policy increasingly relies on surrogates rather than direct US military involvement. He is training Syrian rebels to take on the government of President Bashar Assad and this week launched air strikes to back up Iraqi forces trying to retain the city of Tikrit.
To Obama's Republican critics, he is ceding the traditional US leadership role. The White House denies it is disengaging from the region and says it has been in close contact with the Saudis over their plans in recent days.
Although the Saudis spoke with top US officials as they debated an air assault in support of embattled Yemeni President Abd-Rabbu Mansour Hadi, US officials acknowledged gaps in their knowledge of the kingdom's battle plans and objectives.
Asked when he was told by Saudi Arabia that it would take military action in Yemen, General Lloyd Austin, the head of the US military's Central Command, told a Senate hearing on Thursday he spoke with Saudi Arabia's chief of defence "right before they took action."
He added that he couldn't assess the likelihood of the campaign succeeding because he didn't know the "specific goals and objectives."
Adel al-Jubeir, Saudi Arabia's ambassador to the United States, said Riyadh consulted closely with Washington on Yemen - but ultimately decided it had to act quickly as Houthi rebels moved toward Hadi's last redoubt in the southern city of Aden.
"The concern was, if Aden falls, then what do you do?" al-Jubeir told a small group of reporters on Thursday. "The concern was that the situation was so dire you had to move."
Saudi Arabia's air strikes point toward an aspiration to defend its regional interests with less reliance on the US security umbrella that has long been the main thrust of Washington's relations with the oil-rich kingdom.
Riyadh has been growing increasingly assertive since early 2011, when Washington's reluctance to back former Egyptian leader Hosni Mubarak in the face of mass protests led the Saudis to doubt its commitment to traditional Arab allies.
Obama's decision in summer 2013 not to bomb Syria after the use of poison gas there, coupled with its sudden announcement it had conducted secret nuclear talks with Riyadh's nemesis Iran, further alarmed the Saudis.
"If the operation is successful, I think we will see a major turn in Saudi foreign policy. It's going to be assertive, become more aggressive in dealing with the Iranian expansionism," said Mustafa Alani, an Iraqi security analyst with ties to the Saudi Interior Ministry.
The Obama administration is reluctant to get drawn into direct military action in another Arab conflict when it is already facing daunting challenges in Syria and Iraq.
The worsening Yemen conflict forced Washington to evacuate all remaining US special forces from the country, further undermining the US campaign of drone strikes against the most lethal branch of al Qaeda based there.
Sunni Saudi Arabia's intervention in Yemen is the latest front in a growing regional contest for power with Iran that is also playing out in Syria, where Tehran backs Assad's government, and Iraq, where Iranian-backed Shi'ite militias are playing a major role in fighting.
While US officials have downplayed the scope of the relationship between Iran and Yemen's Houthis, al-Jubeir said that members of Iran's Islamic Revolutionary Guard Corps and Iranian-backed Hezbollah are on the ground advising the Houthis.
One senior US official described Riyadh's operation as a "panic response" to the fast-deteriorating situation in Yemen that the Saudis feared could spill over its border.
The official, speaking on condition of anonymity, suggested that the 10-nation Saudi-led coalition had been patched together so quickly that its effectiveness was in doubt.
The White House says it will not join directly in military operations in Yemen, but has set up a cell to coordinate US military and intelligence support to the operation. But US officials said they were sharing intelligence information on a limited basis so far.
US officials said they discussed the deteriorating situation in Yemen with Saudi Arabia over the course of recent weeks.
Secretary of State John Kerry discussed Yemen at length during a March 5 visit to Riyadh, but it was "not clear (the Saudis) had made any decisions about potential action at that point," said a senior US official who spoke on condition of anonymity.
"We had been talking with the Saudis throughout the course of the last several days about what they were thinking and what type of support we could render with regards to their actions in Yemen," US National Security Council spokeswoman Bernadette Meehan said.