Guns, Oil, and Drugs.
The US military (Guns) blows the shit out of any country that looks to be shifting away from the dollar for sale of Oil, and it's the currency of choice for black markets (Drugs) the world around. That creates, and maintains, an additional demand for the Dollar that keeps it's price from reflecting the supply.
Sorry, but this is bullshit. The dollar has value because people want to hold it. The existence of FOREX nullifies transactional demand as a source of value. Despite the many problems with the dollar and the US economy, all of the others seem to be worse.
You can't buy oil on FOREX.
If you want oil, you can use whatever currency you have to buy dollars on FOREX, use them to buy oil a millisecond later, and the seller can then dump them for whatever currency they want (again on FOREX) a millisecond later.
Perhaps some tiny sliver of value comes from those two milliseconds where people are holding them against their will, but the bulk of the value of any currency comes from people wanting to hold it. No force in the universe, not guns, not oil, not drugs, can compel people to hold a currency they don't want to hold.