President Roosevelt listens as Saudi King Abdul Aziz speaks with him in French aboard a U.S. warship in this Feb. 20, 1945, file photo, in Great Bitter Lake, near Cairo, during a stop on Roosevelt's way home from the Yalta conference. The marriage of convenience was consummated in one war and grew closer in another. Now in a third, in the shadowy struggle against terrorism, America and Saudi Arabia, the odd couple of the desert, are growing apart. (AP Photo/File)

On Tuesday, President Obama will travel to Riyadh to pay his respects to the late King Abdullah and meet the new Saudi king, Salman bin Abdul Aziz. Obama's decision to cut short his trip to India is a good signal of how important the trip is: Thanks to geopolitics and oil, the United States and Saudi Arabia have had an important, if sometimes strained, alliance for decades.

By coincidence, next month also marks the 70th anniversary of the beginnings of that relationship. It began when President Franklin D. Roosevelt was returning from the Yalta Conference in 1945, where world leaders had met to discuss the future of a postwar Europe. On his way home, Roosevelt decided to meet with some of the Middle East and Africa's most important leaders: King Farouk of Egypt, Haile Selassie of Ethiopia and the first Saudi king, Abdul Aziz ibn Saud (the father of both King Abdullah and Salman, plus every other Saudi king).

Roosevelt and King Abdul Aziz met on board the USS Quincy in the Suez Canal on Feb. 14 and talked for several days. It was the first time that Abdul Aziz, a charismatic yet isolationist military leader who had united Saudi Arabia, had left the country: He is said to have brought eight sheep with him on board to be slaughtered for dinners.

William A. Eddy, an American expert on Arab culture who had been acting as the U.S. minister to Saudi Arabia, was present at the meeting. He later published an account, titled "F.D.R. meets Ibn Saud," that gave a rare first-person glimpse of the meetings. It showed the two world leaders building up a remarkable rapport:

The King and the President got along famously together. Among many passages of pleasant conversation I shall choose the King’s statement to the President that the two of them really were twins: (1) they were both of the same age (which was not quite correct); (2) they were both heads of states with grave responsibilities to defend, protect and feed their people; (3) they were both at heart farmers, the President having made quite a hit with the King by emphasizing his rural responsibilities as the squire of Hyde Park and his interest in agriculture; (4) they both bore in their bodies grave physical infirmities–the President obliged to move in a chair and the King walking with difficulty and unable to climb stairs because of wounds in his legs.

According to Eddy, the last point was especially important: Roosevelt ordered one of his two wheelchairs to be given as a gift to Abdul Aziz, who later kept it on display.

The two men went on to talk about a variety of political issues, in particular the plan to find European Jews a new home in Palestine (Abdul Aziz was vehemently opposed). They eventually came to an agreement that centered around U.S. support and military training for Saudi Arabia, then a fledgling country surrounded by stronger nations, in return for oil and political support in the region. "I would take no action, in my capacity as Chief of the Executive Branch of this Government, which might prove hostile to the Arab people," Roosevelt later wrote to the Saudi king in a follow up letter.

Unfortunately, the great friendship between Roosevelt and Abdul Aziz didn't last – the U.S. president died less than two months later. His successor, Harry Truman, didn't always see eye-to-eye with Abdul Aziz (in particular, his views on Israel were far less conciliatory), who himself died in 1953. However, the core themes of the Saudi-U.S. relationship established at that meeting – security and oil – have endured through five Saudi kings and 12 U.S. presidents.

The endurance of that partnership is truly remarkable when you consider all that has happened in that time: The collapse of the Soviet Union (a mutual enemy that was a key driver in the creation of the relationship), Saudi Arabia's growth as an oil superstar, the conflicts between Israel and Arab states, the rise of Islamist extremism, and Saudi Arabia's restrictive domestic policies are just a few issues that might have blown up weaker relationships. How much longer will it last?

Saudi Arabia's King Salman greeted then-President Obama as he arrived with then-first lady Michelle Obama in Saudi Arabia in January 2015. The president cut short a trip to India to pay his respects after the death of King Abdullah. (Reuters)

Correction: This post originally said that Roosevelt died two years after meeting Abdul Aziz. It has been corrected to say two months.