Dubai, Oct 20, 2018 (AFP)

Saudi Arabia has long been a major US ally in the Middle East, thanks primarily to security considerations and oil.

But their diplomatic friendship has not always been a smooth one.

- Partners since 1945 -

The two countries established diplomatic relations in 1940, in the early stages of World War II.

On February 14, 1945 their partnership was sealed during a historic meeting between king Abdel Aziz bin Saud and president Franklin D. Roosevelt on board the cruiser USS Quincy in the Suez Canal.

The agreement saw the US guarantee military protection for the kingdom in return for privileged access to oil reserves, which were discovered in enormous quantities in the 1930s.

- Cooperation over Saddam -

After Iraqi dictator Saddam Hussein's invasion of Kuwait in August 1990, Riyadh allowed hundreds of thousands of US troops to deploy in the kingdom.

The country served as a base for the US-led coalition during the 1991 Gulf War against Saddam.

Over the following years, the coalition continued to station planes in the kingdom to enforce a "no-fly zone" over southern Iraq, provoking the anger of Saudi fundamentalists who carried out two anti-US attacks on Saudi soil in the mid-1990s.

- Post 9/11 tensions -

The September 11, 2001 attacks against the US represented a serious setback for bilateral relations -- 15 of the 19 plane hijackers were Saudi nationals.

Saudi Arabia denounced the attacks, but was accused of quietly financing Islamic extremism.

Riyadh refused to take part in strikes against Afghanistan in late 2001, or participate in the Iraq war in 2003, although the US again used Saudi territory for air operations against Saddam.

Washington evacuated most of its remaining soldiers from Saudi Arabia and transferred them to Qatar, the headquarters for its aerial operations in the Gulf, but maintained military cooperation with Riyadh.

- Confidence crisis under Obama -

Riyadh supported the Syrian uprising against President Bashar al-Assad and did not hide its anger after US president Barack Obama in September 2013 backed away from carrying out air strikes against the regime.

In October that year, Saudi Arabia refused to take a seat at the UN Security Council, to protest against what it regarded as multilateral and US inaction over the Syrian crisis.

The 2015 nuclear agreement with rival regional power Iran -- signed by the US and other world powers -- further undermined Saudi Arabia's confidence in the Obama administration.

- Trump boosts ties -

Delighted to turn the page on the Obama era, Saudi leaders warmly welcomed President Donald Trump's arrival.

In May 2017, Trump was received with pomp in the Sunni kingdom for his first overseas presidential visit.

The US president called for the isolation of Iran, to counter the Shiite power's growing influence in the Middle East.

The US and Saudi Arabia announced huge contracts exceeding $380 billion, including $110 billion for the sale of US arms to Riyadh to conter Tehran and Islamist radicals.

The two allies accuse Iran of arming Huthi rebels in Yemen, where since March 2015 Riyadh has intervened to support the government.

On March 20, Trump gave Crown Prince Mohammed bin Salman an effusive welcome at the White House, hailing a "great friendship".

"Saudi Arabia is a very wealthy nation and they are going to give the United States some of that wealth hopefully, in the form of jobs, in the form of the purchase of the finest military equipment anywhere in the world," said Trump.

On May 8, Riyadh "supports and welcomes" Trump's decision to withdraw from the Iran nuclear agreement.

- Khashoggi affair -

On October 2, Saudi journalist Jamal Khashoggi disappeared after entering the Saudi consulate in Istanbul.

The Washington Post contributor had been critical of the crown prince and lived in self-imposed exile in the US from 2017.

Turkish officials accused Saudi Arabia of a state-sponsored killing.

On October 20, Riyadh admitted that Khashoggi was killed inside the embassy and said he died during a "brawl".

Trump endorsed the explanation as credible and "an important first step", saying the US had not yet completed its own review of the case.

But Saudi Arabia's version of events drew scepticism from some top US lawmakers.