Saudi Arabia’s royal family is frightened — and that’s a problem for the U.S.-Saudi relationship.

The Saudis are surrounded by enemies. To the north, Abu Bakr Baghdadi, leader of Islamic State, has promised to overthrow the Al Saud dynasty, which he calls “the serpent’s head.” To the south, Sunni-led Saudi forces are at war against Shia Muslim rebels in Yemen. To the east, the Al Saud face the rival they fear most, Shia-ruled Iran.

The Saudis have problems at home, too. Fearing subversion from both Islamic State and Iran, the government has cracked down on Sunni and Shia dissidents alike, jailing writers, journalists and human rights lawyers as well as potential terrorists. The plummeting price of oil has blown a hole in the government’s budget while the population, accustomed to subsidized housing and utilities, keeps growing.

And the family faces a succession crisis; 80-year-old King Salman, who ascended to the throne last year, is described privately by diplomats as nearly senile.

Once their regime was a pillar of conservative stability; now fear has made them unpredictable.

On New Year’s Day, the government abruptly announced the execution of 47 domestic prisoners. Although most were Sunni extremists convicted of membership in Al Qaeda, four were Shia Muslim activists, including the country’s most prominent radical preacher, Sheik Nimr al-Nimr. Nimr’s crime was not terrorism, but sedition; he had called publicly for the secession of Saudi Arabia’s Eastern Province, where most of the kingdom’s Shia citizens — and most of its oil reserves — reside.

Nimr’s death set off a cascade of events. Demonstrators in Iran set fire to Saudi Arabia’s embassy in Tehran, unrestrained by riot police. Iran’s supreme leader, Ayatollah Ali Khamenei, said the Saudi leaders would face “divine vengeance.” Saudi Arabia broke diplomatic relations with Iran and demanded that other Arab countries follow suit. (Only a few did.)

In the past, the Saudi princes could usually rely on the United States to take their side in any disagreement with Iran. That’s not what happened this time.

Instead, the White House and the State Department said they were unhappy with both countries — Saudi Arabia for executing Nimr, Iran for failing to protect the Saudi Embassy.

In private, U.S. officials didn’t hide their exasperation at the Saudis — and Saudi officials didn’t hide their unhappiness with the Americans.

It seems telling that Secretary of State John F. Kerry, who scrambled to calm things down, was able to reach Iran’s foreign minister, Mohammad Javad Zarif, almost immediately by phone; Saudi Arabia’s foreign minister, Adel al Jubeir, didn’t accept Kerry’s call until a day had passed.

“We’ve seen a long deterioration in the U.S.-Saudi relationship, and it started well before the Obama Administration,” a former U.S. ambassador to Riyadh, Charles W. Freeman Jr., told me last week.

“The U.S.-Saudi relationship is based entirely on interests, not values,” he said.

And in recent years, the two countries have increasingly seen their interests diverge.

For decades, both countries viewed Iran as an incorrigible threat; yet since last year’s agreement to dismantle Iran’s nuclear program, the Obama administration has sought to enlist Tehran as an arm’s-length partner in ending Syria’s civil war.

For decades, the United States needed Saudi oil; now, thanks to shale oil, the United States has surpassed Saudi Arabia as the world’s largest producer.

For decades, the Saudis counted on the United States to bolster repressive yet stable regimes in the Arab world. But in 2003, the U.S. toppled Iraq’s Saddam Hussein, and in 2011, the U.S. supported a revolution against Egypt’s Hosni Mubarak — in both cases, against the advice of Saudi kings.

In return, Freeman said, the Saudis have increasingly ignored U.S. wishes as they chart their own security course. “Their conclusion is that the only effective thing they can do is go off on their own,” he said. Indeed, U.S. officials urged the Saudis last year not to execute Sheik Nimr, but their advice was ignored.

By the end of last week, that short-term crisis, at least, was subsiding. Saudi Arabia’s defense minister, 30-year old Prince Muhammad bin Salman, the king’s favorite son, said he didn’t want war with Iran. Iran’s reformist president, Hassan Rouhani, said the rioters who attacked the embassy would be prosecuted as criminals.

Kerry said officials from both countries assured him that they still support his effort to convene peace talks between Syrian rebels (backed by Saudi Arabia) and the Syrian government (backed by Iran).

Nevertheless, the fracture in U.S.-Saudi relations isn’t going away, because the foundations of the relationship — the interests the two countries had in common — are no longer as strong.

Even when the two countries were closer, punctilious American diplomats noted that Saudi Arabia was never a formal ally of the United States; the correct word, they said, was “partner.”

Both countries still need each other, but less than before. They’re still partners — but colder, more distant partners now.

Doyle McManus is a columnist for the Los Angeles Times.