The first thing the US did when it invaded and took over Iraq was reverse Saddam Hussein's recently policy of demanding Euros in payment for his countries oil.
Some think that's what the war was all about and that Iran's insistence on receiving Euros for its oil is the real reason that country has been targeted.
Dollars? Euros? What's the difference?
The difference is this...if you are the dominant currency, you can support your lifestyle and war machine by printing more when you run out.
Take that power away and there will be a very economic painful dislocation in the US.