Given Donald Trump's new commitment to support military adventurism by Saudi Arabia in Yemen and more generally against Iran, it might be worth reconsidering how this alliance developed.
The beginning for Saudi Arabia was in 1744 when a wandering radical cleric, Mohammed bin Abdel-Wahhab met up with a local chieftain, Mohammed bin Saud in the village of Diriyah, whose ruins are now located in the suburbs of the current Saudi Arabian capital, Riyadh. Wahhab converted Saud to his cause of spreading the strictest of the four Sunni shari'as, the Hanbali code, throughout the world, and this remains to this day the ideology of the House of Saud, the ruling family of Saudi Arabia, with this ideology widely known as Wahhabism. The territory ruled by the early Saudis expanded to cover a fair amount of the Nejd, the central portion of the Arabian peninsula, but when they threatened control of Mecca in 1818, ruled by Egyptians under the Ottomans who collected the moneys gained from pilgrims visiting there, the Egyptian leader, Muhammed Ali, invaded the Nejd and destroyed Diriyah. The Saud family moved to the next village over, Riyadh, and reconstructed their small state, which expanded again in the mid-1800s, although near the end of the century they were defeated and exiled to Kuwait by the rival Rashid family from Hail to the north of Riyadh.
In 1902 the 27 year old family leader, Abdulaziz bin al-Rahman bin Faisal al Saud, reconquered Riyadh and would eventually establish the modern Kingdom of Saudi Arabia (KSA) through marital and martial conquests, with its modern boundaries established in 1932, and Abdulaziz (known in the West as "Ibn Saud") bearing the title of King and Protector of the Two Holy Places (Mecca and Medina), which he had conqurered in 1924. He would have 43 sons, and today's king, 81-year old Salman, is one of the last of them, and Abdulaziz would die in 1953. It should be noted that Saudi Arabia was independent of the Ottoman Empire, and was one of the few parts of the Muslim world that did not fall under the rule of a European power, along with Turkey, Persia/Iran, and Afghanistan.
In the early years, especially in the 1920s, he sought outside advice and support from the British, especially St-John Philby, the rival at Whitehall of T.E. Lawrence, and the first European to cross the Empty Quarter of the Arabian peninsula. Philby was especially helpful during the revolt by the combined forces of the Rashidi and the Ikhwan (Muslim Brotherhood) whom Abdulaziz managed to defeat in 1929, with the rebels pushing an ultra-fundamentalist line against Abdulaziz (an replay of this revolt occurred 50 years later in 1979, with the Ikhwan seizing control of the Grand Mosque in Mecca for a time). Philby would convert to Islam and take several wives. He was also the father of later Soviet spy, Kim Philby.
The first interest by anybody in the US came out of two agreements in 1928 and 1929, the Red Line Agreement that gave the territories of the former Ottoman Empire to a set of British and French companies, and then the As Is agreement of 1929 between Sir Henri Deterding of Royal Dutch Shell, Baron John Cadman of Anglo-Persian (now BP), and Walter Teagle of New Jersey Standard (now Exxon Mobil) at Deterding's Achnacarry Castle in Scotland. These agreements amounted to an early effort to divide up the oil producing world in a cartel. Out of this, Jersey Standard got Saudi Arabia, although at the time oil had not been discovered there. It would be in 1938 by geologists from Jersey Standard, and agreements for production with cash payments for Abdulaziz in gold bars were made. In 1948, Abdulaziz would become the first leader of an oil-producing nation to succeed in getting a 50-50 profit sharing agreement, and as oil production surged there in the 1950s and after, the money would begin to flow into Saudi Arabia providing the basis for its modernization, even as it retained its highly traditional and strict version of Wahhabist Islam and Hanbali shari'a law code.
While Saudi Arabia initially favored Nazi Germany at the beginning of World War II, much like Iran then, it gradually shifted to the Allied side, with FDR declaring the protection of Saudi oil reserves a US national interest in 1943, and the Saudis officially declaring war on Germany in early 1945. It is widely viewed in KSA that the alliance was sealed in 1945 when FDR was returning from Yalta shortly before his death and met briefly on a boat in the Suez Canal with King Abdulaziz, producing a famous photograph of the two of them smiling and shaking hands, shortly before FDR's death. And indeed, despite some ups and downs, the alliance has held since, with oil at its center.
Given that, the nature of the relationship has changed substantially over time. One major change, signaled initiallly by that 50-50 profit sharing agreement in 1948, was an increase in Saudi control over the oil aspect of it, with OPEC founded in 1960, which would impose a quadrupling of oil prices in 1973 in the wake of the Saudi oil export embargo against the US for the US supporting Israel in the Yom Kippur war of that year. Prior to that embargo, KSA had managed to nationalize ARAMCO, the Arabian-American Oil Company, which produced the oil in Saudi Arabia, the original owners of ARAMCO being Jersey Standard, New York Standard (Mobil, now merged with Exxon), Texaco, and California Standard (now Chevron). These companies, especially Exxon Mobil, continue to have an active relationship with ARAMCO, but the Saudis have been in control of their oil and their oil industry since the beginning of the 1970s. This shifted the relationship to being one more of the US becoming the protetctor of KSA, providing it with arms as the petrodollars poured in, and this aspect of the relationship has reached a new height with this latest visit and arms deal, arranged by former Exxon Mobil CEO and now SecState, Tillerson.
It is worth noting also that for most of the postwar period probably the major irritant in the Saudi-US relationship has been Israel, which even now KSA does not recognize, and Trump's flight from Riyadh to Tel Aviv was the first such direct flight on that route ever. Israel supporters for many years complained about "Arabists" in the US State Department who were more oriented to worrying US oil interests in the Middle East and especially in Saudi Arabia. But today there is now an alliance of convenience between KSA and Israel in their mutual dislike of Iran.
Which brings us to the current situation. I personally think that the current Saudi leadership has gone off the rails in their anti-Iran attitudes. The differences are both sectarian and ethnic, Sunni versus Shi'i Islam and Semitic Arabs versus Indo-European Iranians, with this manifesting itself in a regional power struggle. But this is a relatively recent conflict, only getting going since the 1979 Islamic Revolution in Iran, and only getting really hot with the overthrow of Saddam Hussein by the US under George W. Bush. It was the Saudis who convinced Bush's dad not to go to Baghdad to overthrow Saddam in the 1991 Gulf War, arguing that he kept a balance of power as a Sunni Arab leader against Iran. And they argued with Bush, Jr. not to go in for the same reason, although they would support the US effort modestly once it happened, even though it aggravated Osama bin Laden and al Qaeda against the Saudi monarchy for supporting the US so openly (even though the US had supported the decision by then Saudi intel chief, Turki bin Faisal, to send bin Laden to Pakistan to aid in the anti-Soviet campaign in Afghanistan). But the replacement of a Sunni-led regime in Iraq by a Shi'i led one supported by Iran has upset the Saudis greatly. They also do not like Iranian support of Assad in Syria, who appears to have won his war against largely Sunni rebels, many of them supported by KSA, and now the Saudis are bogged down in a war in Yemen against local Zaydi Shi'a, whom they claim (not with full credibility) are being supported by Iran. So they, and the Israelis, want the US to join them in an anti-Iran crusade.
I think we are at a dangerous moment here. The nuclear deal with Iran is the most importantdeal that Obama made, and even the Saudis and Israelis know it. What they do not like about it is that it meant that the economic sanctions on Iran were relaxed. But most of those sanctions were only put on to get Iran to the nuclear negotiating table. There is no way they can be reimposed without Iran returning to having a nuclear program. The most influential person in KSA now appears to be the son of King Salman, 31-year old Mohammed bin Salman, Deputy Crown Prince and Defense Minister, who gets lots of good press in the US. But for all the talk of reform, he has not moved to let women drive or to desegregate workplaces by gender. He seems to be a warmongering hothead who has pushed this so far fruitless and destructive war in Yemen, which has led to incipient famine in that nation as well as its likely falling apart into pieces. He has even talked about "taking the war to Iran," which we can only hope that he will not be tempted to do with all those fancy arms that he is buying from the US. Trump, or whoever is in charge of US foreign policy in the near term, will really have to both defend the nuclear deal with Iran and resist this warmongering push by our longtime erstwhile ally. Let us hope that this is done.