A critical alliance: the long, close relationship between the U.S. and Saudi Arabia boils down to the importance of oil.

• Author: Smith, Patricia

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Sailing home from the Yalta Conference in the closing months of World War II, President Franklin D. Roosevelt made a detour that would have an impact on U.S. foreign policy for decades to come.

His unexpected stop was a meeting with Saudi Arabia's King Abdulaziz. In 1945, Saudi Arabia was little more than a desert. So why did Roosevelt bother?

Because eight years earlier oil was discovered there, and FDR knew that a secure supply of oil was critical to America's war effort and future growth.

OPEC

Six decades later, the two countries remain dependent on each other: The U.S. needs Saudi oil, and Saudi Arabia needs American political and military support.

As one of the most powerful members of the Organization of Petroleum Exporting Countries (OPEC), the Saudis have enormous leverage over the supply and price of oil. And though it is no longer the world's largest oil producer (it's now Russia), Saudi Arabia remains the largest oil exporter.

Politically, Saudi Arabia shares U.S. concerns over the growing power of Iran, which has been ruled by radical anti-American clerics since 1979. Iran has defied the U.N. by pursuing a suspected nuclear-weapons program.

During the 1991 Gulf War, after Iraqi leader Saddam Hussein overran neighboring Kuwait, the U.S. sent 500,000 troops to Saudi Arabia to protect it from an Iraqi invasion. (This was during the first Iraq war, as opposed to the second Iraq war, which began in 2003.)

The U.S. troop presence angered some Saudis and became a...