Live updates: Follow the latest on Trump's Gulf trip
The relationship between the US and Saudi Arabia is as old as the kingdom itself.
Even before King Abdulaziz Ibn Saud established the Kingdom of Saudi Arabia in 1932, the US had joined the UK in extending diplomatic recognition to the new ruler.
The heart of the relationship was oil, with the world realising fossil fuels would drive 20th-century economies, and with Saudi Arabia an expected source of vast reserves.
A year later, in 1933, a concession agreement was signed between Standard Oil of California and Saudi Arabia to create the California Arabian Standard Oil Company.
Oil was discovered in 1938, with the venture renamed Aramco, or the Arab American Oil Company, and in 1988, Saudi Aramco, now one of the largest and wealthiest companies in the world.
Although Saudi Arabia remained neutral during all but the last months of the Second World War, bombing attacks by Fascist Italy on targets in the Arabian Gulf led Washington to become increasingly concerned about the vulnerability of such a strategic economic asset.
By 1943, US President Franklin Roosevelt was offering King Abdulaziz military assistance, which was formalised in an agreement signed by the two leaders in 1945 on an American warship moored in the Suez Canal.
Long-standing ties
The deal would see the US provide military protection for Saudi Arabia, with American oil supplies guaranteed in return. The creation of this long-standing security agreement began with the training of Saudi troops and the establishment of American airbases in the kingdom. In 1963, US President John F Kennedy sent a squadron of fighter jets to protect Saudi Arabia from attacks by Yemen during its civil war.
Washington and Riyadh have long shared similar concerns about global and regional stability and the dangers of extremism, whether political or religious. The influence of the Soviet Union in the Gulf topped the list in the 1960s and 1970s, to be replaced by the rise of groups including Al Qaeda and ISIS in more recent decades.
The fall of the Shah of Iran in 1979 caused the US to lose its other main regional ally, with military aid significantly increased to Saudi Arabia. This was despite tension created by American support of Israel during the 1973 Arab-Israeli War, when an oil embargo was declared by Opec on the US, and petrol prices surged.
One aspect of the embargo, and the rapid rise of oil prices, was that Saudi Arabia’s income rose steeply, and when the embargo was lifted the following March, the kingdom embarked on a major purchase of weapons, worth $2 billion to American companies.
The relationship has endured despite challenges such as America’s close relationship with Israel, which led the US Congress to vote against arms sales to Saudi Arabia several times in the 1980s.
The First Gulf War that followed the invasion of Kuwait in 1990 led King Fahd to authorise the posting of more than 600,000 US troops to Saudi Arabia. While Kuwait was swiftly liberated, the arrangement saw increasing discontent among extremists, including Osama bin Laden, who had fled to Taliban-controlled Afghanistan to establish a global terrorist network, with the US and Saudi Arabia as its targets.
21st-century relations
This came to a head on September 11, 2001, when it emerged that 15 of the 19 hijackers responsible for the worst terrorist attack on American soil were from Saudi Arabia.
The complexity of Saudi-US relations was illustrated when prominent Saudi citizens, unconnected with the attacks, were allowed to return home despite the closure of American airspace and the hostility of many ordinary Americans towards the kingdom.
The attacks by Al Qaeda underlined the importance of the relationship between the two countries and the joint threat they shared, strengthened by a series of terrorist incidents inside Saudi Arabia.
The nuclear deal reached with Iran by Barack Obama, US president at the time, saw the relationship with Washington weaken over concerns about Tehran's regional ambitions, but ties were strengthened during the first term of Donald Trump and the reaffirmation of America’s desire for joint security and economic co-operation.
In 2017, Mr Trump made Riyadh his first stop overseas after becoming president, where he touched a glowing orb to launch Saudi Arabia’s new Global Centre for Combating Extremist Ideology.
The war in Gaza has been another test, with the high civilian death toll during Israel’s onslaught on the enclave, effectively ending any prospect of Saudi Arabia joining the Abraham Accords brokered by Mr Trump during his first term.
Mr Trump's decision to revisit Riyadh so early in his second term is a reminder that, after nearly a century, the relationship with Saudi Arabia remains important to Washington.
