Federal Reserve

The petrodollar system. While a fiat currency, the Fed can print at will, the dollar is informally tied to a commodity: oil. That's what keeps inflation from skyrocketing.

"From 1972 to 1974, the U.S. government made a series of agreements with Saudi Arabia. These agreements created the petrodollar system.
The U.S. government chose Saudi Arabia because of its vast petroleum reserves, its dominant position in OPEC, and the (correct) perception that the Saudi royal family was corruptible.
In essence, the petrodollar system was an agreement that the U.S. would guarantee the survival of the House of Saud. In exchange, Saudi Arabia would:

1. Use its dominant position in OPEC to ensure that all oil transactions would happen in U.S. dollars.

2. Invest a large amount of its dollars from oil revenue in U.S. Treasury securities and use the interest payments from those securities to pay U.S. companies to modernize the infrastructure of Saudi Arabia.

3. Guarantee the price of oil within limits acceptable to the U.S. and prevent another oil embargo by other OPEC members.

Oil is the world’s most traded and most strategic commodity. Needing to use dollars for oil transactions is a very compelling reason for foreign countries to keep large U.S. dollar reserves." 

Thus the alliance with the Saudis.