The battle for dominance in the Middle East draws on ancient enmity between the Shiite and Sunni branches of Islam, but what drives it is a thoroughly modern struggle for power between Iran and the Arab monarchies led by Saudi Arabia.
Now the United States, which has generally sought to gloss over Sunni versus Shiite conflicts, has been pulled into the tumult as the fight plays out on battlefields along a 2,000-mile arc from northern Syria through Iraq to Yemen.
U.S. forces were on both sides simultaneously Thursday.
In Iraq, U.S.-led fighter jets, bombers and drones launched a second day of airstrikes to support Iraq's mostly Shiite military, backed by Iran, which had stalled in an offensive aimed at ousting Sunni militants from the strategic city of Tikrit.
In Yemen, the U.S. provided intelligence and logistical support to Saudi Arabia as it carried out airstrikes aimed at dislodging Shiite Muslim rebels, backed by Iran, who have overrun much of the Sunni-dominated country.
To make the situation even more complex and fraught, the military actions come as the United States and five other global powers scramble to meet a Tuesday deadline for negotiating a preliminary deal with Iran aimed at preventing it from someday building nuclear weapons.
Iranian President Hassan Rouhani, in a letter to President Obama and leaders of the five other countries, criticized the Saudi airstrikes on Thursday. But he did not suggest they would stand in the way of a nuclear deal.
The strikes were "exacerbating the crisis," Rouhani said in a Twitter message describing his letter. White House officials said they would not comment on the letter but did not dispute Rouhani's account.
Despite the confusing array of nominal alliances, Middle East experts see a consistent pattern of the United States coming to the aid of established governments against rebels, both Sunni and Shiite, who threaten stability.
"The U.S. is pursuing a steady line of defending governmental power in the region, the existing national shape of the region," said
Vali Nasr, dean of the Johns Hopkins School of Advanced International Studies in Washington.
In Iraq, where the government is battling Islamic State militants, and Yemen, which is under siege from Houthi insurgents, the Obama administration is opposing forces that "are pushing those countries into becoming failed states," Nasr said in a telephone interview from Dubai, United Arab Emirates.
Washington has provided overhead surveillance, intelligence and logistics support for the Saudi-led intervention in Yemen, officials said. A joint command center has been set up in Riyadh, the Saudi capital, for U.S. and Saudi military personnel to plan and launch the strikes.
The decision to back Saudi efforts in Yemen serves in part to reassure Saudi Arabia and the smaller Persian Gulf Arab states that are historic American allies. They see the Shiite threat on the Arabian Peninsula as existential, especially now as, in their view, Iran has been gaining influence in the region.
The U.S. negotiations with Iran over the nuclear program, the Pentagon's tacit military cooperation with Iran in the fight against Islamic State in Iraq, and White House reluctance to get more involved in the Iran-backed war in Syria had raised doubt in Sunni Arab capitals about whether U.S. policy was tilting in Iran's favor.
"It had looked like the U.S. was swinging away from the Sunni Arabs despite all the denials," Nasr said. The intervention in Yemen "gives a sense to the region now that the U.S. is firmly backing" the Sunni Gulf countries, he said.
The U.S. has plenty of reasons to want to reassure those allies. For example, Saudi Arabia pumps enough oil to make possible the international sanctions that brought Iran to the negotiating table.
The United States also has an imperative to prevent Yemen from becoming a lawless state where Al Qaeda in the Arabian Peninsula, the most dangerous Al Qaeda franchise, could flourish.
U.S. officials have worked hard to find a diplomatic solution to the many-sided civil war in Yemen. They played down, until recently, the extent of Iranian military support for the Houthis.
Now the U.S. also stands to gain by giving very public support to Saudi Arabia as it takes on the Houthis.
In a region where symbols are important, it was widely noted that the normally press-shy Saudi ambassador, Adel Jubeir, broke the news about the airstrikes in Washington, not back home. He briefed reporters again Thursday.
Jubeir said the Saudis, the Persian Gulf states and the U.S. had discussed intervening in Yemen for some time, but the decision to attack was made Wednesday when officials saw the Houthis advancing quickly on Aden, the country's economic hub.
"The concern was the situation was so dire, you had to move," Jubeir told reporters Thursday.
Jubeir said Saudi Arabia does not want to start a regional war, even as he listed half a dozen Sunni countries, from Qatar to Egypt, that had offered to join the raids, he said.
"Iran is playing a destabilizing role in the region," Jubeir said, pointing to the presence of Iranian weapons and military advisors in Lebanon, Syria, Iraq and Yemen.
There's another potential plus: U.S. support for Saudi airstrikes in Yemen could make Saudi rulers more willing to accept an Iranian nuclear deal, said Rep. Adam B. Schiff (D-Burbank), the top Democrat on the House Intelligence Committee.
The Saudis see U.S. backing in Yemen "as an indicator of our willingness to push back against Iranian efforts to increase hegemony in the region [and] that may influence how comfortable they are with a nuclear agreement," Schiff said. "It is very important for the U.S. to have Saudi Arabia's back when it comes to Yemen."
At the same time, however, the U.S. interventions in both Yemen and Iraq could toughen opposition to the deal from Iran's hard-line mullahs and Revolutionary Guard Corps generals, who see U.S. involvement in the Middle East as the chief threat to Iran's aspirations as a regional power.
In Iraq, the U.S. military has sought to keep its distance from Iran and the Shiite militias it backs.
Before the U.S. agreed to help Iraqi forces reclaim Tikrit, U.S. commanders insisted that thousands of Iranian-backed Shiite militia fighters who were part of the attacking force leave the area, Gen. Lloyd Austin, the top U.S. commander, said Thursday.
"I will not — and I hope we will never — coordinate or cooperate with Shiite militias," Austin told the Senate Armed Services Committee.
Shiite militia commanders insisted they were leaving the battlefield to protest the U.S. role, not to accommodate U.S. demands.
Either way, the failure of the Iran-backed offensive to retake the city created an opening for the U.S. to come to the rescue of its Iraqi allies, giving Washington a rare propaganda victory even while the battle for Tikrit goes on.
Austin said the offensive had failed because "the Iran approach was taken" and the forces "were poorly led."
Pentagon officials have been privately chagrined that Iranian Gen. Qassem Soleimani initially appeared to lead the Tikrit attack. U.S. officials say he orchestrated deadly attacks on American troops in Iraq in 2007 and 2008.
Sen. John McCain (R-Ariz.), chairman of the Armed Services Committee, blamed Iran for most of the region's troubles.
"In Iraq, the same Iranian-backed [Shiite] militias that killed hundreds of American soldiers and Marines are dictating the battle plans of the Iraqi government and exacerbating the sectarian tensions that first led to the rise" of Islamic State, he said. "In Syria, the Iranian-backed Assad regime together with Iranian proxies like Hezbollah continue the slaughter that has killed more than 200,000 Syrians and displaced 10 million more."
Secretary of State John F. Kerry raised the airstrikes in Yemen in a meeting Thursday with his Iranian counterpart, Foreign Minister Mohammad Zarif, according to Jeff Rathke, a State Department spokesman.
"But let me stress, this was not and is not the focus of the talks," he added. "The focus remains squarely on our and the international community's concern over Iran's nuclear program."
Josh Earnest, the White House press secretary, said U.S. support for the air attacks in Yemen should have no effect on the nuclear negotiations with Iran. But he said a successful nuclear deal "won't eliminate the other long list of concerns that we have about Iranian behavior."