In 1971 OPEC had a meeting at which they decide that whereever oil is bought or sold, it may only be bought or sold in US dollars. This means that Mexico selling oil to China has to be sold in US dollars, Holland trading with Morrocco has to be done in US dollars, etc etc. What this means is for anyone to trade oil, they have to buy dollars. This is how the US can owe more money than any country in the history of the world, but doesn't have to pay anything back because since 1971, thanks to this OPEC agreement, the US effectively has a magic chequebook.
Imagine you're maxed out past your overdraft limit in every bank in the world, and have been for decades, but it doesn't matter because everyone still accepts your cheques and they never come back to the bank.
So what could possibly happen to the magic chequebook of the US dollar to bring all of that money back to chase Washington on Wall Street? Well, it almost happened.
On 30/10/2000, when a switch was made to a deposit account in the Wall Street branch of a French bank. This was the account handling the 2.3 million barrells of oil sold per day by Iraq under the "oil for food" scheme. The Iraqis said that they wanted to switch the account from being a dollars denominated account to a euro denominated account. The UN couldn't stop them, but it looked like a stupid thing to do at the time because the euro was only worth eighty cents to the dollar. They'd lose money on every barrell they sold. They'd bankrupt their country within a year. The Iraqis didn't care, they hated America so much that they didn't want to trade in their currency.
In 2001, the euro gained 25% against the dollar. The Iranians then decided to switch their central bank's reserve funds from dollars to euros too. This makes them member number two on the axis of evil list, with number one being the Iraqis who started the trend.
7/12/2002, North Korea declares that it's going to do ALL of it's trading in euros. Not just oil, but everything. They're quickly branded "Axis of evil" member number three.
In Venezuela, Hugo Chavez, the elected president, gains chainmanship of OPEC. He calls a meeting in Spain, in April of 2003 and on the table is the proposal that every single OPEC member stops trading in dollars and starts trading in euros. If that happened, that would be the federal reserve's worst nightmare because then every single central bank in every single country in the world has to abandon the dollar and start trading in euros. Such is the need for oil. All the dollars in those banks would be flushed out, the market would be awash with dollars and it would become a worthless currency. Worth less than toilet roll. The US would be back in it's 1920s and 1930s depression and this time they wouldn't have the Nazi party to invest in to save themselves.