When did the alliance start?
The relationship goes back to the late 1930s, just after Abdul Aziz ibn Saud consolidated squabbling Arab tribes into a kingdom. U.S. energy companies had discovered oil in the Arabian Peninsula, and they asked their government to promote their interests with the new monarch. In 1945, President Franklin Roosevelt met King Abdul Aziz aboard a U.S. ship in the Suez Canal, and the two got along famously. FDR gave the ailing king one of his own wheelchairs, which the king later called his "most precious possession." FDR succeeded in ensuring that the U.S., and not the British, would control Saudi oil. In return, the U.S. would provide security for the kingdom: Within a few years, a U.S. military base was set up near the oil fields. Over the decades, the oil-for-security arrangement has become vital to both countries. Saudi Arabia is now the U.S. defense industry's largest foreign customer, buying some $112 billion worth of weapons during the Obama administration alone.

Has the alliance ever wavered?
The 1973 oil embargo was a major rough patch. For a year, the Saudis quit selling to the U.S. in retaliation for U.S. support of Israel in the Yom Kippur War. But the two countries made up, united in opposition to the Soviet Union. Even the 9/11 attacks couldn't loosen the bond. Al Qaeda leader Osama bin Laden and 15 of the 19 hijackers who flew planes into the World Trade Center and the Pentagon were Saudi nationals, and U.S. public opinion turned strongly against the kingdom after Saudi citizens were allowed to leave the U.S. right after the attack — before the FBI could interview them. But President George W. Bush, whose family had long-standing Saudi business relationships, stood by the alliance, and in 2005, he was photographed holding hands with then–Crown Prince Abdullah. In the decade after 9/11, the Saudis spent more than $100 million on public relations in the U.S., trying to overcome the country's image as an exporter of terrorism.

Is that image true?
Yes. Decades ago, the Saudi monarchy made a tacit bargain with radical Islamists in the country: It would fund the spread of Wahhabism, the Saudi form of ultraconservative Islam, and jihadism around the world, as long as the radicals didn't blow up targets inside Saudi Arabia. Saudi money funded Islamist militants in Afghanistan, Pakistan, Bosnia, and the Russian province of Chechnya. After 9/11, Saudi officials claimed to have turned off the money spigot. But secret U.S. diplomatic cables published by WikiLeaks in 2009 said Saudi Arabia "remains a critical financial support base" for al Qaeda, the Taliban, the Pakistani terrorist group Lashkar-e-Taiba, and the Palestinian terrorist group Hamas, giving them "millions of dollars annually."

What about human rights?
With its draconian form of sharia law, Saudi Arabia's autocratic government is consistently rated among "the worst of the worst" human rights offenders. Its gender apartheid system treats women as second-class citizens — shrouded in abayas, dependent on male guardians, and mostly barred from going out alone and from any form of public life. There's no freedom of religion, and the press is censored. Brutal, public floggings and stonings are the penalty for such crimes as adultery and apostasy. Those arrested are routinely tortured to extract confessions. Last year, Saudi Arabia put to death 146 people for crimes including murder and drug dealing; most of the executions were beheadings.

What's in it for the U.S.?
Saudi oil, of course, although last year it made up only 9 percent of what the U.S. used, because of our fracking revolution. More strategically important today is the Saudis' regional role in counterbalancing Iran. Ever since the 1979 Islamic Revolution, when Iranian mullahs took U.S. diplomats hostage, the U.S. has seen Iran as the most dangerous actor in the Middle East. Saudi Arabia, which practices Sunni Islam, opposes the Iranian Shiite theocracy's proxy interventions in other Middle Eastern countries, including Syria, Lebanon, and Yemen. More recently, the Saudis have begun working with America's other major regional ally, Israel, because both countries see Iran as an existential threat.

How has Trump affected the relationship?
The president has long-standing business ties with the Saudis; by his own account, he's sold them millions of dollars' worth of real estate. "Am I supposed to dislike them?" he asked while campaigning for president. "I like them very much." Since taking office, he has made the Saudi alliance a priority; his first foreign trip was to Riyadh. Trump's son-in-law Jared Kushner quickly grew close to one of the king's sons, Mohammed bin Salman, and the administration strongly supported Mohammed's elevation to crown prince last year, viewing him as a reformer intent on modernizing his country. Congress, though, is not so enamored. The killing of Jamal Khashoggi in the Saudi consulate in Istanbul last month prompted the Senate to invoke the Global Magnitsky Human Rights Accountability Act, which requires the president to identify within four months which individual Saudis should be sanctioned. "In moments like this, you have to embrace your values," said Sen. Lindsey Graham (R-S.C.). "No more transactional interactions."

U.S. support for the war in Yemen
Barack Obama initially backed Saudi Arabia's war against Iran-backed Houthis in Yemen in 2015 in order to prevent the overthrow of the Yemeni government. But after thousands of civilians were killed in Saudi airstrikes, Obama suspended a sale to the Saudi military of some $390 million in weaponry. Trump pushed that sale through right after he took office, and U.S.-made laser-guided bombs are now being used against Houthi militants and Yemeni civilians. The Pentagon is also giving the Saudis intelligence help in identifying targets, and U.S. planes provide midair refueling for Saudi aircraft. Since last year, U.S. special forces have been stationed on the Saudi-Yemen border to help the Saudis destroy Houthi missile sites. This support, though, may soon end, as hunger and chaos threaten millions of Yemeni civilians. "Now is the time to move forward on stopping this war," Defense Secretary Jim Mattis said last week.