HARI SREENIVASAN: But, first, President Obama today held his second annual summit with Saudi Arabia and its Gulf Cooperation Council allies, Kuwait, Bahrain, Qatar, Oman, and the United Arab Emirates.

They discussed common interests and issues, but beneath the show of unity lies rising tensions between the U.S. and Saudi Arabia.

Chief foreign affairs correspondent Margaret Warner has the story.

MARGARET WARNER: Beneath the ornate chandeliers of Riyadh's Diriyah Palace this morning, Saudi King Salman gave flicker of a smile, as President Obama outlined steps the U.S. and Gulf states were taking to deepen cooperation on regional and security challenges.

But their two days of meetings came amidst new strains emerging in the decades-long partnership between the two countries. The fundamental bargain, oil for guns, a reliable supply of Saudi oil, in return for U.S. military protection and weaponry for the kingdom.

But, today shale-oil-and-gas-rich America has less need of Saudi oil. And with the region in turmoil, from conflicts in Syria, Iraq and Yemen, Islamic State aggression, oil prices falling, and a rising Iran, the original bargain is fraying.

RANDA SLIM, Middle East Institute: This relationship saw its glory days in the 1980s, 1990s. And since the al-Qaida attacks in New York on September 11, 2001, a lot of mistrust has seeped into this relationship, and that breach has been overcome.

MARGARET WARNER: Randa Slim, an analyst at the Middle East Institute, says each country feels let down by the other.

RANDA SLIM: The GCC, led by the Saudis, has not done enough in the fight against ISIS. One factor is that they consider the major fight for them is Iran, and not necessarily ISIS. The Saudis look at the Obama administration as being enthralled with Iran, and not willing to stand up to what they perceive to be subversive Iranian activities in their backyard.

MARGARET WARNER: President Obama sought to downplay those differences today.

PRESIDENT BARACK OBAMA: A lot of the strain was always overblown. During the course of our administration, the GCC countries have extensively cooperated with us on counterterrorism, on curbing the financing of terrorist activities. They are part of the ISIL — counter-ISIL coalition.

MARGARET WARNER: But the kingdom has a list of grievances that have shaken their confidence, chief among them, the U.S. did nothing to save its longtime Sunni partner, Egyptian President Hosni Mubarak, from being toppled in the Arab Spring revolt.

BRUCE RIEDEL, Former CIA & National Security Council Official: The fall of Mubarak sent the message to the Saudis that the United States won't stand by autocrats when the people of the country is demanding change. It was a shock to the Saudis.

MARGARET WARNER: Former CIA and National Security Council official Bruce Riedel spent decades working in the Middle East.

BRUCE RIEDEL: From the Saudis' standpoint, they immediately said, what about us? Are we going to go the same way?

MARGARET WARNER: Then, last year, Riyadh felt betrayed by the Obama administration's nuclear deal with its Shiite regional rival, Iran.

BRUCE RIEDEL: The Saudis and the other Gulf states are paranoid that the United States is going to shift sides, and that we're going to find a new friend in Iran and dump them.

MARGARET WARNER: And, lately, the kingdom is angry about legislation making its way through Congress to let 9/11 victims' families sue Saudi Arabia, home to 15 of the 19 hijackers.

Washington has grievances too, top on the list, that Saudi Arabia's focus on provoking its archrival, Iran, as it did when it beheaded a Saudi Shiite cleric in January, fuels sectarian tensions in the region. That makes it harder to unite against America's number one foe, the Islamic State.

After joining the U.S.-led anti- ISIS coalition in 2014, the Saudis made bombing runs against the terrorist group in Iraq and Syria, but those have stopped. Instead, Riyadh is using billions of dollars in U.S. plane and weapons sales to fight a proxy war against Iranian-backed Houthi rebels in Yemen.

SEN. CHRIS MURPHY (D), Connecticut: The humanitarian casualties of this war have been catastrophic.

MARGARET WARNER: Democratic Senator Chris Murphy of Connecticut, a member of the Senate Foreign Relations Committee, has introduced a bill to put conditions on U.S. arms sales to the Saudis, saying their use undercuts America's image and interests.

SEN. CHRIS MURPHY: Inside Yemen, this isn't a Saudi bombing campaign. This is a U.S.-Saudi bombing campaign.

MARGARET WARNER: Because it's done with American weaponry.

SEN. CHRIS MURPHY: It's done with American weaponry. It's done with American targeting. It's done with American refueling of planes.

MARGARET WARNER: And he faults the Saudis for letting ISIS and al-Qaida actually expand in Yemen.

SEN. CHRIS MURPHY: We also know for a fact that the Saudis have not been targeting either of those groups inside this civil war.

MARGARET WARNER: President Obama expressed impatience with Riyadh's rivalry with Iran in a recent interview with "The Atlantic" magazine, saying Saudi leaders "need to find an effective way to share the neighborhood and institute some sort of cold peace with Iran, not count on the U.S. to back them in endless proxy wars."

Saudi leaders made it clear that comment didn't sit well with them. U.S.-Saudi strains are most pronounced over handling the civil war in Syria with its multiple players, Iranian-backed President Bashar Assad, the Islamic State and other terror groups, and Western-backed rebels.

Saudi Arabia has funded the training and equipping of anti-Assad rebels, but, of late, not done much more to defeat ISIS there.

RANDA SLIM: Saudi Arabia doesn't believe this is going to be achieved without removing Assad from power, and that is where the goals differ.

MARGARET WARNER: And then there is the greatest, but rarely spoken of point of friction, the Saudis funding of Sunni mosques and schools that spread their fundamentalist strain of Islam, Wahhabism, throughout the Muslim world.

SEN. CHRIS MURPHY: If you want to look at the roots of al-Qaida, if you want to look at the roots of ISIL, there's no way of denying that part of those roots run through the export of Wahhabism from Saudi Arabia.

MARGARET WARNER: Yet, with the two nations still needing each other, as long as the Saudi monarchy is in power, no one predicts a breakup any time soon.

For the "PBS NewsHour," I'm Margaret Warner.