What is the cause of the strong relationship between America and Saudi Arabia?

Rachel Bronson's "Thicker than Oil:America's Uneasy Partnership with Saudi Arabia" provides a good look at how the relationship developed. Saddam's invasion of Kuwait, Iran-Contra, the rise of Muslim fundamentalism, the seeds of 9/11 sown at the end of World War II: each turns out to be the logical effect of a cause put into play over many years by presidents, kings, generals, entrepreneurs and ambassadors, all appropriately greased by oil, money and a mutual distaste for communism. Bronson follows the trail, adds the insights and uses the voices of the people who were actually there to document the U.S.-Saudi partnership over the last sixty years. Bronson starts with the questions, how could two countries as different as America and Saudi Arabia forge such a close alliance for so long? The first response is the alliance has not been all plain sailing. Over the years, America and Saudi have clashed repeatedly, not least over America's position on the Israeli-Palestinian conflict. Dispelling the myth that America and Saudi Arabia have always been close, Bronson pulls together the different strands of the story and highlights the conditions under which the two states have been attracted to one another pointing out the alliance was always about more than oil. Anti-communism and real-estate were equally important factors that brought the two countries together. America's anti-Soviet agenda found an natural partner in a devout country that was awash with money; time and again, America would turn to Saudi Arabia to finance anti-communist struggles the world over. The Saudis often obliged, for their own anti-communist reasons. Saudi Arabia's attractive location also led policy makers as early as World War II to pronounce the fruits of partnership with the kingdom. From "oil, gold and real estate" a strong alliance emerged, one that went awry after September 11. For many Americans, this is not an alliance worth saving. Since then, other factors have also come into play, based largely on Saudi Arabia's geographic location, its importance within the Islamic world and obviously oil.