The basic framework was strikingly simple. The U.S. would buy oil from Saudi Arabia and provide the kingdom military aid and equipment. In return, the Saudis would plow billions of their petrodollar revenue back into Treasuries and finance America’s spending.

It took several discreet follow-up meetings to iron out all the details, Parsky said. But at the end of months of negotiations, there remained one small, yet crucial, catch: King Faisal bin Abdulaziz Al Saud demanded the country’s Treasury purchases stay “strictly secret,” according to a diplomatic cable obtained by Bloomberg from the National Archives database.

With a handful of Treasury and Federal Reserve officials, the secret was kept for more than four decades—until now. In response to a Freedom-of-Information-Act request submitted by Bloomberg News, the Treasury broke out Saudi Arabia’s holdings for the first time this month after “concluding that it was consistent with transparency and the law to disclose the data,” according to spokeswoman Whitney Smith. The $117 billion trove makes the kingdom one of America’s largest foreign creditors.