The Dollar is not entirely fiat.
The explanation that the dollar is held up by "faith in government" is only partly correct. Americans use the dollar because our Government tells us to, and we have faith that our Government will turn its guns against us if we don't.
But this does not explain why foreign nations and multinational corporations, much more powerful than you or I, hold to the dollar; yet the Dollar is the international reserve currency. Even though there is also a power disparity between the American Government and the governments of other countries it is not so great as to allow it to force its currency on the whole world without powerful allies. This begs the questions: Who are these allies and what do they gain?
The most important is OPEC. Like I said the dollar, though not backed by gold, is still internationally commodity backed. OPEC sells exclusively for the Dollar. After the collapse of Bretton Woods system the dollar switch from a gold coupon to an oil coupon. We are in the era of the Petro-Dollar.
If you need oil, you need dollars. Any oil importing country must be a dollar importing country. In this way America is the center of world trade, and in a position to also be the toll collector of world trade. In this 20th century iteration of triangle trade goods are exported to the US for Dollars, the Dollars are then brought to OPEC for Oil.
The American Navy rules the seas. For this reason, other important allies are the former naval powers: Japan and the UK. They are committed to maintaining the status quo of a friendly imperial power controlling the seas. Thus Tony Blair is explained.
Those benefited by the Petro-Dollar are the owners of the Federal Reserve, the owner of the Dollar. Enlisted to this cause is the Military Industrial Complex, which defends the Dollar's status as an oil coupon, in exchange for some of those freshly printed coupons.
The Oil Tyrants that rule the OPEC nations are pivotal to the system, but stand to gain little directly; they could just as easily sell their oil for gold. So in addition to the threat of "Regime change" they are bought by bribes and favors: a carrot and a stick.
So the American tax payer and the Military Industrial Complex again come into play, Oil Tyrants get guns and wars courtesy of Uncle Sam. Saddam gets war against the Iranians. Iran gets guns to defend themselves from Saddam. The Saudis and Kuwaitis get war against Saddam and weapons against the Israelis.