Why Are Saudi Arabia And The U.S. Allies?

The U.S. and Saudi Arabia have managed a strong alliance despite clear differences in values. So why are the dissimilar nations so close?

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Beyond oil: the US-Saudi alliance, explained (vox.com)
"When Saudi Arabia executed 47 people, including dissident Shia cleric Nimr Baqr al-Nimr, seemingly in a deliberate effort to inflame tensions around the Middle East, it led a number of Americans to ask a question: 'Why are we allies with these people?'"

The United States Shouldn't Choose Saudi Arabia Over Iran (politico.com)
"Only two Muslim powers remain standing in the Middle East, and suddenly they are on the brink of war.

Financing Terrorism: Saudi Arabia and Its Foreign Affairs (brownpoliticalreview.org)
"They cross over during the Hajj."

Oil Embargo, 1973-1974 (history.state.gov)
"During the 1973 Arab-Israeli War, Arab members of the Organization of Petroleum Exporting Countries (OPEC) imposed an embargo against the United States in retaliation for the U.S. decision to re-supply the Israeli military and to gain leverage in the post-war peace negotiations."