There's no doubt that it began with oil. In 1933, the Saudi monarchy granted the American company Standard Oil exclusive rights to look for oil in the country's eastern province. In 1938, the joint US-Saudi venture, eventually called ARAMCO, found truly staggering reserves.

The US government wanted to protect its companies' investment, especially when America was in dire need of crude during World War II. In 1943, FDR declared the security of Saudi Arabia a "vital interest" of the United States � despite the Saudis' official neutrality in the Axis-Allies conflict.

After the war, the Saudis began pushing for a bigger role in ARAMCO � and eventually took it over, though they didn't nationalize it outright until 1980. But the US-Saudi alliance deepened even as America's direct role in the Saudi oil sector waned. That's because both countries agreed on what they saw as the dominant Middle East issue of the time: Soviet influence in the region.

The nature of the Saudi system made the country a natural enemy of Soviet communism. The Saudi government is both a monarchy and a theocracy: The Saudi royal family ceded power over certain sectors, like the judicial system, to Sunni clerics from the hardline Wahhabi sect. The Soviet policy of supporting Marxists and nationalist movements in the Middle East posed a direct threat to both halves of the Saudi government: the monarchs feared leftist revolutionary sentiment while the Wahhabis loathed Soviet secularism.

The US and Saudi Arabia, allying themselves against this shared enemy, expanded their initial oil relationship into a more expansive security alliance. In 1951, the US and Saudi Arabia established the Mutual Defense Assistance Agreement, the first formal defense agreement between the two nations. It provided for American arms sales to Saudi Arabia and American training of the Saudi military.

US policymakers concluded "that religion could be a tool to staunch the expansion of godless communism," the Council on Foreign Relations' Rachel Bronson writes. And that led them to the Saudis: "The Eisenhower administration had hoped to make King Saud (1953�1964) into a globally recognized Islamic leader and transform him into 'the senior partner of the Arab team.'"

This relationship grew strong enough to survive some serious disputes, most notably the 1973 oil embargo on sales to the United States (put in place to protest US support for Israel during the Yom Kippur war that year). US-Saudi cooperation, as my colleague Max Fisher explains, was pretty extensive. It included "facilitating contacts between the CIA and religious pilgrims visiting Mecca from heavily Muslim Soviet Central Asia" as well as "sending Saudi Arabia's formidable intelligence service to work alongside US, British, and French agents in Muslim parts of Africa to undermine Soviet influence there."

But perhaps the single most important event in the Cold War�era history of this alliance was the 1979 Soviet invasion of Afghanistan. Saudi policymakers worried this was the first step toward a broader Soviet push into the Middle East. The US saw it as an opportunity to bog down its rival in its own Vietnam War. The two countries teamed up to covertly ship weapons to the anti-Soviet mujahideen rebels.

The program was enormous � according to Bronson, "the United States and Saudi Arabia each spent no less then $3 billion, channeling assistance to armed, anti-US Islamic fundamentalists." This required tight intelligence coordination between the two countries. Here's how the New Yorker's Steve Coll describes it in his authoritative book Ghost Wars:

[Saudi Prince] Turki [al-Faisal] reached a formal agreement with the CIA in July 1980 to match US congressional funding for the Afghan rebels. Each year the Saudis sent their part of the money to their embassy in Washington. The Saudi ambassador in Washington, Bandar bin Sultan, then transferred the funds to a Swiss bank account controlled by the CIA. The agency used its Swiss account to make its covert purchases on the international arms markets. Langley's Near East Division, which handled the Saudi liaison, had to continually haggle with Turki's GID over late payments.

This brought a new era of cooperation between the two countries: Generally speaking, the US doesn't serve as a money launderer for foreign powers unless it really thinks it's important.

How the US-Saudi alliance grew and changed after the Cold War

[caption id="" align="alignnone" width="800"] (Joyce Naltchayan/AFP/Getty Images) 1997: Then-President Bill Clinton meets with then-Defense Minister Sultan bin Abdul Aziz.[/caption]

By 1990, the Berlin Wall had come down and Saudi Arabia had nationalized ARAMCO, seeming to pull away at much of the foundations of the US-Saudi alliance. But something else happened that year that would bring the two countries together in new and often uncomfortable ways.

That year, Iraqi leader Saddam Hussein invaded Kuwait in a naked grab for Kuwait's productive oil industry. This terrified the Saudis: They shared a long border with Iraq and, of course, huge oil reserves. The US response was immediate: On August 7, just five days after Saddam first moved into Kuwait, America sent troops to Saudi Arabia to defend the kingdom from Iraqi attack. In January 1991, US troops attacked Iraqi forces, pushing Saddam out of Kuwait and forcing him to sue for peace.

But a funny thing happened after the war: The American troops stayed. The US had had a small training force in Saudi Arabia since the '50s, but this was stepped up dramatically after the Gulf War. About 5,000 US troops remained in the kingdom (they mostly left in 2003), tasked with maintaining a no-fly zone over southern Iraq as well as defending several key Saudi facilities from possible Iraqi aggression.

This US deployment wasn't just about Saddam. It was about signaling a broader US commitment to Saudi security and defending the Middle East status quo even after the Soviet threat had receded.

The post�Cold War order in the Middle East was generally a good one for the United States. Most major powers � Saudi Arabia, Israel, Turkey, Jordan, and Egypt � were American allies. Demonstrating that these states were backed by American military might, US policymakers believed, would help ensure that oil kept continued to flow into international markets and would help prevent destabilizing new wars.

9/11 and the US-Saudi alliance in the era of al-Qaeda

[caption id="" align="alignnone" width="800"] (David Hume Kennerly/Getty Images) George W. Bush meets with Saudi Foreign Minister Saud al-Faisal on September 20, 2001.[/caption]

The US-Saudi relationship took a major shock after 9/11, and not just because 15 of the 19 hijackers were Saudi nationals. For decades, Saudi money had funded extremist groups and education centers around the world as means of keeping the Wahhabi authorities (whom the House of Saud still relied on for political legitimacy) happy. (This was also part of the US-approved strategy in the 1980s to promote the mujahideen in Afghanistan.)

This funding helped create fertile soil for al-Qaeda to generate and spread fundamentalist views that were even more extreme than those of Wahhabism. Osama bin Laden, himself Saudi, got direct support from the Saudi government during the 1980s Afghanistan War � though Saudi Arabia would revoke his citizenship in 1992 after he denounced the monarchy as insufficiently Islamic.

It seemed fair, at least in part, to blame the attacks on Saudi Arabia's foreign policy, and it seems that many Americans did. In a 2001 Zogby poll of Americans before the attack, 56 percent said they viewed Saudi Arabia favorably. In December 2001, that number fell to 24 percent.

But the crisis was, in a different way, an opportunity. Bin Laden was, if anything, more of a threat to Saudi Arabia than he was to the United States. His fundamental goal was to topple the Saudis and other Middle Eastern dictatorships; attacking the US was only a tactic designed to pressure the "far enemy" into leaving the Middle East. The US and Saudi Arabia, once again, had an enemy in common.

As a result, the old US and Saudi intelligence relationship from the Afghanistan War kicked into overdrive. It's true that Bush administration officials did criticize Saudi support for extremism, and the US funded limited democracy promotion programs in Saudi Arabia. But counterterrorism cooperation was the top priority: Bush was unwilling to jeopardize a valuable security relationship by fundamentally challenging Saudi rule or its relationship with the Wahhabi authorities.

Once again, the US's perceived short-term interests in an alliance with Saudi Arabia trumped any qualms about the nature of Saudi Arabia's regime and its policies.

The relationship's fundamental problems won't go away

[caption id="" align="alignnone" width="800"] (Albin Lohr-Jones/Pacific Press/LightRocket/Getty Images)) Anti-Saudi protestors in New York after the Nimr execution.[/caption]

Despite the longstanding nature of the US-Saudi alliance, the relationship still has problems � ones that have come to the fore in the wake of Nimr al-Nimr's execution over the new year.

The Saudis, for their part, are paranoid about the United States. Texas A&M's F. Gregory Gause, an expert on Saudi Arabia, writing in an essay for�Lawfare, blames this on a "structural" problem in relationships between stronger and weaker states. The Saudis, according to Gause, are afraid of both "abandonment" (the US withdrawing its support) and "entrapment" (having to suffer the consequences of American mistakes in the Middle East). This means that no matter what the US does, Saudis are still likely to worry. That's the nature of being allied with a stronger power.

In recent years, Saudi fears have swung decisively toward abandonment. As the Obama administration reaches out to Iran, Saudi Arabia's leading rival in the region, the Saudis have become scared that the US going to sell it up the river. Gause explains:
In the past, when Washington was more bellicose toward Iran, the Saudis worried that they would pay the price of Iranian retaliation for any U.S. attack on Iran. Now, with Americans and Iranians sitting at negotiating tables with each other, Saudi elites worry that their interests will be neglected, if not actively sold out, by their US ally. In October 2013, the chairman of the foreign affairs committee of the Saudi Consultative Council, the appointed and non-binding Saudi version of a parliament,said�"I am afraid there is something hidden�If America and Iran reach an understanding, it may be at the cost of the Arab world and the Gulf states, particularly Saudi Arabia."
This speaks to an even more fundamental problem for the US-Saudi alliance: It's only as durable as the shared interests between the countries. US relationships with countries like the UK and Japan are about much more than interests and common enemies � they're about shared values, histories, and visions for the world. The US can't have that kind of alliance with an ultraconservative authoritarian theocracy. The Saudi alliance is fundamentally transactional: It's only as strong as the benefits both countries get out of it.

America's role in the Saudi oil sector, the Soviet Union, and Saddam Hussein are gone; jihadism is still around, but it's not clear how long that will remain an issue that actually unites the two powers. To take one example, the US's top priority in Syria is defeating ISIS, while Saudi Arabia's is toppling Bashar al-Assad (an Iranian client). This leads the two countries to see issues such as the value of peace talks and arming the Syrian rebels very differently.

Indeed, when the countries' interests are in conflict, each tends to go its own way. This is exactly what happened with the Nimr al-Nimr execution: Saudi Arabia executed him�in order to stoke anti-Shia sentiment, a crude attempt to shore up its support among domestic Sunnis and regional Sunni powers. From the Saudi point of view, these benefits outweighed the damage done to its relationship with Shia Iran and America's efforts to coordinate a Syria peace deal and region-wide front against ISIS. The United States, needless to say, sees things differently, and is thus quite upset about Nimr's execution.

So the key question coming out of recent Saudi behaviors is this: Are the common interests that bind the United States and Saudi Arabia together more important than the conflicting interests pushing them apart? The answer will determine just how strong this decades-old alliance will be in the coming years.

This article was written by Zack Beauchamp for Vox on Jan. 6, 2016. Zack writes about all of the things that are not American things. He previously edited a section on political thought at ThinkProgress and, before that, contributed to The Dish. It's pronounced BEE-chum.